Introduction

The Ecdysozoa can be broken into two main groups; animals in the Panarthropoda and those that aren't. Unfortunately there isn't really a convenient a name to define the latter, other than Non-panarthropods which is a bit cumbersome. Whatever we call them they include the nematodes and their close cousins the nematomorphs and a series of animals with introverts and scalids: Priapulida, Kinorhyncha and Loricifera. The last two are small and found in the meiofauna.

The usual alpha- chitin cuticle that we associate with the ecdysozoa is modified in the nematodes and nematomorphs. Instead of chitin strands embedded in a proteinaceous matrix these two phyla appear to have replaced the chitin strands with collagen. Chitin has been detected but it no longer appears to be important in the structure of the cuticle.

Nematoda

Nobody knows how many nematode species there are and part of the problem is that most nematodes look very much alike. New molecular techniques for identification of different species will no doubt see the numbers explode beyond the few that we know from either the medical or economic importance in agriculture and forestry. The similarity works to our benefit because once you've seen one nematode you've pretty much seen them all. These are pseudocoelomate animals and when you look at them end on they have an almost triradiate symmetry reflected in the structure of the pharynx, and internally by a circular tripartite brain that surrounds the pharynx. Nematodes keep their pseudocoelomic fluid under constant pressure my contraction of the longitudinal muscle lining the body wall. As a result the pharynx has two valves to facilitate the ingestion of food. The only muscles associated with the pseudocoel are, as mentioned, longitudinal and the fluid filled hydrostatic skeleton of the pseudocoel results in the characteristic whip like motion. The muscle organization is unusual and the muscle cells extend to the nerve cord, rather then the usual seen in other animals where the nerve cord extends to the muscles that they innervate. That's not all that's unusual about the group. They have renette cells presumed excretory and ameboid sperm! They're dioecious and the male genital pore is at the posterior end of the animal and often armed with spicules that anchor the male inside the midventral genital pore of the female.

Nematomorpha

Like the nematodes, nematomorphs have a collagenous cuticle, only longitudinal muscles, and fluid filled pseudocoelom acting as a hydrostatic skeleton. You guessed it, they also have that same whip-like movement when they swim. One difference is these are much larger animals. Adults in horse troughs were often confused with the long hairs from a horse's mane or tail and its how they get their common name of the horsehair worms. Another difference is that these ecdysozoans are arthropod parasites; insects on land or freshwater environments and crustacea in the oceans. Their life cycle includes a larval stage that hatches from eggs laid by the female and invade an intermediate aquatic host ultimately consumes by the arthropod definitive host. Indirect development distinguishes them from the nematodes along with rod shaped rather than ameboid sperm and the absence of the muscle extensions to the nervous system. As we've seen in other internal parasites there often isn't need for a digestive tract when you can absorb sufficient nutrients directly from the host. The nutrient rich hemolymph of arthropods that surrounds the parasite inside the hemocoel allows these animals to survive as adults without a digestive tract. Chalk up another difference with the nematodes.

Scalidophora

These three phyla all share the characteristics of having a introvert and scalids surrounding it and together are referred to as the Introverta. Two of the three phyla in the taxon are small and found in the meiofauna the other, the priapulids are much larger - although there are small versions as well. Priapulids burrow in the sediments and capture small invertebrates ingested as the introvert is pulled back in. The priapulids have a large body cavity but there no agreement about whether it's a pseudocoelom or a true coelom. The fluid in the cavity is filtered by protonephridia and they're usually associated with the pseudocoelomic condition as is the use pseudocoelomic fluid being the main circulatory fluid. The fluid contains wandering amebocytes and the respiratory pigment hemerythrin and is a part of the hydrostatic skeleton used to extend the introvert. Priapulids have a a larval stage, indirect development, that resembles the loriciferans. That's not the only link to the other two phyla. Some species of priapulids are also small and part of the meiofauna where you'll also find the Kinorhyncha and Loricifera.

The Kinorhynch body is divided into two main parts, a head and a trunk of a dozen zonites. The cuticle is arranged in plates with two roof like tergites on the dorsal side and one sternite on the ventral. It gives the animals a triangular appearance in cross section. With the appearance of plates is also the use of muscle bands for movement and each of the zonites articulates against the next. The fluid filled pseudocoelom acts as a hydrostatic skeleton and also helps to extend the introvert when these small animals feed. Along with a pseudocoel and protonephridia to filter the fluid and brain that surrounds the pharynx and connects with a ventral nerve cord. Development is direct and from the fertilized egg hatches a small kinorhynch that adds additional zonites with each moult until all eleven are complete, after that the zonites themselves grow with each moult.

In the transition from priapulids to kinorhynch we've seen the body wall change from muscular, the articulating plates. In the Loricifera the body wall is completely fused into the lorica that covers these small animals and gives them their name. This is another of these phyla that have only been found recently, the other were the Cycliophora, and their discovery hints at the diversity of the meiofauna that may as yet have not been adequately studied. In these animals the introvert is complex with an array of scalids surrounding it. There's no doubt it's a scalid covered introvert but in these animals it no longer inverts, it retracts. The fluid in the body cavity is filtered by protonephridia, suggesting that the space is a pseudocoelom, but no one really sure.

Part of the problem with the group is their small size and what is bound to have been a series of convergent evolutionary traits associated with miniaturization. It is interesting to note that the Introverta remain as a taxon in both molecular and morphological interpretations which suggested that they themselves are monophyletic. What does change in the two interpretations is whether the nematodes and nematomorphs are their sister group or not.

Resources

Keywords

BIO2135 - Pseudocoelomates: Aschelminthes, Cellular constancy, Coelom, Complete digestive tract, Cuticle, Eucoelomate, Eutely, Hydrostatic skeleton, Intestine, Longitudinal muscle, Muscle arms, Pharynx, Protonephridia, Pseudocoel, Pseudocoelomates, Rectum, Renette cells, Roundworms.

BIO3334 - Nematoda, Nematomorpha, Priapulida, Kinorhyncha and Loricifera: Alpha-chitin, Collagen, Convergent evolution, Cuticle, Definitive host, Direct development, Ecdysozoa, Eucoleomate, Eutely, Hemerythrin, Hemocoel, Hemolymph, Hydrostatic skeleton, Indirect development, Intermediate host, Introvert, Lorica, Meiofauna, Pseudocoelomate, Scalids, Syncytium, Triradiate pharynx, Zonites.

Readings, handouts and handouts

Pechenick, Biology of Invertebrates. Chapter 16 and 17
Lecture slides in 3/page and 6/page (~400 Kb)

*Not all of the lecture slides may be presented in class! This handout is in the Adobe Acrobat PDF format. Use this link to get a copy of the Acrobat Reader from Adobe (It's free!)