7: The Common Invertebrate Phyla

Introduction

The animals “without backbones” is a common way to describe the invertebrates, but we will see that it is much more interesting (and instructive) to see what these incredible groups of animals do have, as opposed to what they lack. When we examine the invertebrates, we see more than simply a bunch of animals with which we may or may not be familiar. What we can observe if we look carefully is the evolution of the animals as a whole. Over 95% of species on the planet “lack a backbone”, so get ready to meet not only your ancestors, but the rulers of the planet, both on land and in the sea.

Activity

Using the guidebooks and key, you will examine members of every phylum and class listed below. Explore the living specimens in the seawater tables and the preserved and dried specimens on the side and back of the lab. Get to know the unique characters of each phylum and spend time looking at the animals with your own eyes and using the microscopes. Try to draw as many as you can to practice your observation skills. At the end of the lab, you should not only be familiar with each phylum but also know the main characteristics of the groups.

Major Animal Phyla

Phylum Porifera. The primitive nature of sponges is evident in their body plan, which lacks tissues or organs. The basic body plan of sponges consists of two layers of cells supported by fibers and secreted mineral elements that form a middle layer, called the mesohyl, between the cell layers. These cells and their products form the body wall of the sponge and are organized around a central cavity called a spongocoel. This space is neither a digestive tract nor a body cavity; rather it is simply part of a system of passageways that allow water, drawn across the wall of the sponge to move toward an opening near the top of the sponge, the osculum, thus enabling filter feeding. Special cells, called choanocytes, pump and filter the water of food while also exchanging gases and wastes.

A- Kelvin Song; B- Nick Hobgood. Both CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Phylum Cnidaria This phylum includes anemones, corals, jellies and box jellies, which until recently were thought to be only slightly more advanced than sponges; but given the presence of tissues, they are classified among the metazoans—animals with well-defined tissues and organ systems. This view was developed on the basis of their body plan, which exhibits a tissue level of organization but lacks the organs seen among all remaining animal phyla. Two radially symmetric body forms are seen among most cnidarians: the sessile, cylindrical polyp form characteristic of anemones and corals, and the umbrella-like medusa, a free-swimming form, common to jellies. Among the cells making up the epidermis are specialized stinging cells called cnidocytes, which contain a stinging organelle, called a nematocyst. When stimulated, the nematocyst will evert from the cnidocyte, stinging its prey and, in many cases, delivering immobilizing toxins. These stinging cells are a distinguishing feature of Cnidarians.

Jellyfish by Kevin Raskoff. Collage by Frédéric Ducarme, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons. Hydroid by Bernard Picton, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

Phylum Platyhelminthes includes the flatworms. They are all quite flat (most less than 1 mm thick) and come in many wonderful colors. They often have eye spots and can be seen “gliding” around, as opposed to wriggling about. They glide around on mucus and cilia and can have very fine control over where they are going. These are the first animals with heads and are efficient hunters. They do not have a complete gut, so that food goes in and undigested material comes back out the same opening, which is usually found on the belly on the worm.

OJJ, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

Phylum Bryozoa and three other small phyla have a feeding structure called a lophophore, consisting of a set of horseshoe-shaped or circular ciliated tentacles surrounding the mouth. The cilia create currents of water that pass over the tentacles, where they can trap food particles. They usually live in large colonies made up of very small individuals, like small apartments in a big complex.

A- Cecchetto M, Lombardi C, Canese S, Cocito S, Kuklinski P, Mazzoli C, Schiaparelli S (2019) The Bryozoa collection of the Italian National Antarctic Museum, with an updated checklist from Terra Nova Bay, Ross Sea. ZooKeys 812: 1-22. https://doi.org/10.3897/zookeys.812.26964, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons. B- Reverter-Gil, O., Souto, J. 2019. Watersiporidae (Bryozoa) in Iberian waters: an update on alien and native species. Marine biodiversity. 49: 2735–2752. DOI: 10.1007/s12526-019-01003-4, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons.

Phylum Annelida consists of worms, but this group has one very obvious difference: their bodies are segmented, both internally and externally. Annelids evolved to burrow into the oceanic sediments where they could ingest the substrate and digest the organic detritus that had accumulated there. In general, annelids are elongate, protostomous, schizocoelous, worm-shaped animals divided into a series of similar segments. They have the tube-within-a-tube body plan where each segment has a well-developed coelom, digestive, circulatory, excretory and nervous system. There are three classes of annelids, broadly distinguished by their respective habitats. Free-living marine annelids exhibit the greatest diversity; however, freshwater and terrestrial classes are also well represented. Very few are parasitic.

Class Polychaeta - Annelids with distinct heads bearing sensory organs and a pair of lateral flap-like extensions of the body wall called parapodia bearing many bristles (setae) on each body segment that are involved in locomotion and gas exchange.

© Hans Hillewaert. Creative Commons Attribution-Share Alike 4.0 International.

Phylum Mollusca is a big group of bilaterally symmetric, soft-bodied animals. Molluscs have well-developed organ systems for feeding, digestion, circulation, respiration, excretion, movement, nervous coordination, and reproduction. The molluscan body plan has large anatomical regions: a head, a muscular foot, a visceral mass, which is covered by a mantle that may secrete a calcareous shell. Among the unique and defining features of all molluscan classes, except the bivalves, is the presence of a feeding structure known as the radula; this structure takes on many different forms according to the feeding strategies of each group.

Class Polyplacophora - These are chitons, molluscs with somewhat dorso-ventrally flattened bodies with a reduced head and the shell divided into eight valves which lie dorsally. The chitons are entirely marine and live in intertidal areas that are subject to wave action and alternate drying and submerging caused by tides.

Class Gastropoda – Commonly known as snails, limpets, conchs, and whelks, the gastropods are the largest class in the phylum. Most shelled gastropods undergo torsion and spiraling during embryonic development; this 180-degree rotation of the visceral mass results in the digestive and nervous systems having roughly a U-shape. Spiraling involves the coiling of the visceral mass inside the shell so that the anus and mantle cavity lie in the anterior end at the opening of the shell.

Class Bivalvia – This group includes clams, oysters, mussels, and scallops. They are sedentary, filter feeders characterized by a laterally compressed body surrounded by a two-piece shell consisting of right and left valves. The shell is lined by the mantle and the head is indistinct. A large, muscular foot can be extended and is used in burrowing.

Class Cephalopoda – Commonly known as nautiluses, squids, and octopuses, these animals are active marine predators. Nautiluses have an external shell, but in the others, the shell is internal or absent. The muscular foot has grown up around the head and developed into eight or more long, prehensile tentacles. The well-developed head has accommodating, image-forming eyes.

PaleoNeolitic, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons.

Phylum Nematoda are very small (usually) cylindrical worms that lack any distinct head or other body parts. They don’t have fine control over their movements and are usually seen thrashing about. They have a complete gut and eat just about everything. What they lack in size and complexity, they make up for in numbers. On just about every living thing and drop of dirt, there are nematodes

Bob Goldstein, UNC Chapel Hill http://bio.unc.edu/people/faculty/goldstein/, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons; CSIRO, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons.

Phylum Arthropoda is the largest group of animals on earth, with features such as jointed, hard exoskeletons, with segmentation, and many paired appendages. All members have exoskeletons and grow by the process of ecdysis, or molting. They usually have distinct body regions, such as a head, thorax, and abdomen. This body plan affords the members of this phylum with a tremendous potential to evolve because its simplicity is easily adaptable to gathering food, feeding, locomotion, and sensing. Furthermore, the exoskeleton—formed from a chitinous cuticle and then calcified in aquatic arthropods—provides protection. Soft, pliable regions of the exoskeleton form the basis for jointed appendages, allowing movement.

Subphylum Crustacea – About 40,000 species of lobsters, crabs, shrimps, barnacles, and water fleas. Mostly aquatic arthropods, except pill bugs (terrestrial), in which the body is segmented into two or three parts with biramous (branching), paired appendages that are often highly specialized. The head has well-developed paired appendages, including two pairs of sensory antennae (anteriormost), followed by three or more pairs of mouthparts, including hardened jaws (mandibles). Walking legs are present on the thorax, and, unlike insects, crustaceans have appendages on the abdomen.

Public domain, via Wikimedia Commons.

Phylum Echinodermata This phylum includes about 7,000 species of slow moving or sessile marine animals exhibiting spiny skin. They are voraceous feeders and the adult bodies exhibit secondary, pentamerous radial symmetry, meaning that the body is arranged into five (or multiples of five) repeating units radiating from a central area. The use of the term secondary in this context arises from the fact that the larval stage of these organisms is bilaterally symmetric. The adults have a calcareous endoskeleton composed of bone-like flat plates (ossicles) from which spines emerge, with their bases articulating with the ossicles. Echinoderms have a unique physiological system called a water vascular system that serves a variety of functions, the most prominent being locomotion whereby the system ends in a number of tube feet. They have a complete tubular digestive system, and the body surface (along with some surface specializations) serves as a site for the exchange of respiratory gases and nitrogenous wastes since they lack any other specialized organs to perform these functions.

Class Asteroidea – Sea Stars
Echinoderms are flattened and starlike with arms bearing grooves and tube feet on the oral surface.

Class Ophiuroidea - Brittle Stars and Basket Stars
Echinoderms with stellate bodies that are flattened with long, thin, flexible arms sharply set off from the central disc. Tube feet are present but lack suckers.

Class Echinoidea - Sea Urchins and Sand Dollars
Echinoderms with globular or disc-like bodies in which the ossicles are fused to form a solid endoskeleton (test). All have many movable spines and tube feet.

Class Holothuroidea - Sea Cucumbers
Echinoderms with soft bodies that have become elongate (sausage-shaped) in the oral-aboral axis and the mouth is surrounded by feathery tentacles. There are five rows of tube feet, and the ossicles are greatly reduced.

Class Crinoidea - Sea Lilies and Feather Stars
Echinoderms are characterized by having each arm is divided into two or more long, flexible extensions. Sea lilies have a stalked body and are sessile, while feather stars can swim freely.

PbsouthwoodShokaG.P. Schmahl, Sanctuary Superintendent. Credit: NOAA/NOS/NMS/FGBNMS; National Marine Sanctuaries Media Library.Sharon Mooney (see User:Edwardtbabinski)Daniel Hershman from Federal Way, US© Hans Hillewaert / CC-BY-SA-3.0Michael WolfDerivate Author: Veronidae, CC BY-SA 2.5 <https://creativecommons.org/licenses/by-sa/2.5>, via Wikimedia Commons.

Phylum Chordata Chordates include about 50,000 species of animals ranging from simple invertebrates, such as sea squirts, to relatively complex animals like birds and mammals. All members of the phylum share the following characteristics at some time in their life cycle:

1. All have a notochord at some stage in their development
2. All have a dorsal hollow nerve cord at some stage in development
3. Pharyngeal gill slits appear during development or in adults
4. All have a post anal tail

The phylum Chordata is split into three subphyla: the Urochordata (about 3,000 species of tunicates and salps), Cephalochordata (23 species of lancelets), and Vertebrata (about 47,000 species of fish, amphibians, reptiles, birds, and mammals). The first two subphyla contain animals that lack a backbone, but have notochords to provide some support. They are the invertebrate members of the phylum. In many respects, they more closely resemble the organisms we have studied thus far, even though they are much more closely related to the vertebrates with which we are more familiar. In vertebrates, a segmented column of bones, the vertebral column, replaces the notochord.

Subphylum Urochordata (or Tunicata) “Sea Squirts and Salps”.
Depending on the species, these marine invertebrates are sessile filter feeders (sea squirts) or drifting, pelagic filter feeders (salps and larvaceans) that may form colonies several meters long. The nerve cord and notochord are present only in the tail of the tadpole-like larvae and disappear in the adult.

GFDL, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons. Nhobgood, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons.  U.S. Geological Survey/photo by Dann Blackwood (USGS), Public domain, via Wikimedia Commons

Taxonomic Key to the Major Invertebrate Phyla

Most taxonomic keys are “dichotomous” (two branches), which is to say they are written with a series of two choices to be made about the anatomy of an animal you are looking at. Keys are not made to be read from start to finish like a book or a poem. In each numbered question, you should read both choices, determine which choice best applies to the specimen you are looking at, then go where the key tells you to go, often skipping other steps in between that don’t apply.

1. Radial symmetry or………………………………………………………..…..……….2

Bilateral symmetry………………………………………….….…………………………4

2. Highly porous surface, not true tissues…………………….…………Phylum Porifera

Surface is not highly porous, true tissues present…………….…………………………..3

3. Exhibits pentaramous symmetry and tube feet…………….…Phylum Echinodermata

Lacks pentamerous symmetry and tube feet, possesses tentacles

with nematocysts ……………………………………………..…...........Phylum Cnidaria

4. Macroscopic colony of sessile, microscopic individuals, individuals

small in individual boxes………………………………………………..Phylum Bryozoa

Solitary or colonial in form, individuals of the colony › 0.5 mm in size……………..…….…5

5. Gelatinous…………………………………………………………………….………..6

Not gelatinous……………………………….………………………………….…………7

6. Solitary individuals with 8 rows of comb plates……………..……Phylum Ctenophora

Solitary and/or colonial with incurrent and excurrent siphons, and a

gelatinous exterior called a tunic.………………………..............……..Phylum Chordata

7. Possesses segmentation…………………………………………………….…………..8

Lacks segmentation………………………….…………..…………………….………….9

8. Exoskeleton with jointed appendages…………………………….Phylum Arthropoda

No exoskeleton, appendages, if present, not jointed, segmented, worm-like

body, possibly in a tube (if in a tube, may have tentacles)…………….Phylum Annelida

9. Possesses a foot, radula, arms and/or shell………………..…….……Phylum Mollusca

Lacking all of above, dorso-ventrally flattened or round in cross section………………10

10. Dorso-ventrally flattened to a thickness of less than 1mm ...Phylum Platyhelminthes

Round in cross section, worms, usually very small, some large............Phylum Nematoda

VOCABULARY for INVERTEBRATE KEY

appendages Any part of an animal coming from the main body or trunk, such as arms, legs, antennae

asymmetry Having no symmetry

bilateral symmetry Having a body displaying two similar halves.

colonial A group of organisms of the same species living together.

dorsoventrally From back to front.

exoskeleton An external skeleton, shell.

gelatinous Looks like jelly.

nematocyst The stinging “cells” of cnidarians.

pentaramous symmetry Divided into five parts.

porous Full of tiny holes.

radial symmetry Having similar parts radiating from a central point.

radula A tongue-like toothed structure used in chewing and rasping.

segmented The division of the body into similar parts.

sessile Attached to one place.

siphon An extension of the mantle in molluscs for drawing water into the mantle cavity.

solitary By oneself.

tentacles Long cylindrical tubes for sensory reception or food capture.