1) Body cavity lined, but probably not peritoneal (coelomate or pseudocoelomate)

2) Cuticle that is molted

3) Radial intraepidermal nervous system

4) Protonepridia associated with the gonads (urogenital system)

5) Caudal appendage for gas exchange

6) No circulatory system

7) Complete gut

8) Evertable introvert and pharynx

8) Separate sexes with loricate larvae

9) Marine benthic burrowers

SHOW OVERHEAD PAGE 364 BRUSCA & BRUSCA

Priapulid worms are weird anyway you look at them. They have a bunch of abnormal traits. There are only about 15 different species-hardly any species at all compared to other phyla, but incredible diversity of body shapes among them. This is sort of like gnathostomulids. Anytime you see incredible diversity among a few species, suggests that they once were much more numerous and evolutionarily successful, but represent only a remnant of that diversity today.

The name priapulid comes from Priapos, the Greek god of reproduction and means penis in greek.

SHOW OVERHEAD PAGE 263 CONWAY MORRIS?

1) The first thing you notice about priapulids, is that they do not say pseudocoelomate. This is one of the most debated phyla among taxonomists. Since they have been associated with acoelomates, pseudocoelomates, and true coelomates alike.

Libby Hyman in the 50s categorized them with the pseudocoelomates. Shapeero in 1961 claimed the cavity to be a true coelom, and it was moved to the coelomates. Shapeero claimed the mesenteries suspending the organs were true mesodermal tissue, and therefore it was a coelomate.

For 20 years, Priapulids were coelomates, then Malakhov in 1980 challenged Shapeero’s view saying there wasn’t any mesodermal mesentery.

The mesenteries that suspend organs are actually formed from extracellular material produced by coelomocytes or amoebocytes that move freely in the pseudocoel. These amoebocytes produce fibers that suspend the digestive system and other organs in the fluid-filled space-and the muscle nucleii embedded in the digestive system (endoderm) are positioned to make the mesenteries look like cells. Thus, priapulids are pseudocoelomates, not true coelomates.

Whatever the nature of the fluid-filled cavity, it is certain that it functions as a hydrostatic skeleton.

They bear several other affinities to the pseudocoelomates.

2) They have a cuticle that is periodically molted. This cuticle is composed mostly of chitin and tends to be less flexible than that found in nematodes.

The body covering may also possess spines, warts, or tubercles.

3) The nervous system is interesting in that it is buried within the epidermis and is radial in design (DRAW ON BOARD). A central nerve ring is found in the center. Only is Echinoderms do you find anything similar-which is interesting because there is a radial nervous system in a bilaterally symmetrical animal. I’d tell you about sensory organs but there is little if any information on them.

4) Priapulids are among the simplist animals to have integrated excretory and reproductive systems.

5) Gas exchange probably occurs through the caudal appendage. These structures have a lumen that is continuous with the internal body cavity of the pseudocoelom. Not all pripulids have a caudal appendage. It is prevalent only in those that live in marginally anoxic sediment.

6) Priapulids lack a circulatory system, but they do possess a respiratory pigment in the pseudocoel called hemerythrin.

7) Priapulids have a complete gut. They are benthic marine burrowers. Most are predators that move through soft sediment and feed on sedentary polychaetes and other small burrowing marine invertebrates.

Others are deposit-feeders.

Priapulids have an evertable introvert and proboscis that they extrude during feeding to capture prey. It is usually lined with cuticular teeth for gripping.

Organic feeders have plates for grinding tissue, similar to the gizzard of oligochaete worms.

No one knows anything about how or where digestion occurs in priapulids.

8) Priapulids have separate sexes and fertilization is either internal in small priapulids, or external in larger species.

SHOW OVERHEAD OF PRIAPULID BURROWING (PAGE 356)

I’ve mentioned quite a few infaunal or burrowing invertebrates. Several of them have an evertable introvert (Loriciferans, priapulids, kinorhyncs). These may represent convergent evolution living in soft sediment, or the trait may represent shared anscestry.

How are introvert-like structures used in burrowing?

Early studies were limited due to poor methods of measuring external movement, and not being able to see the animal in the goopy sediment.

That has changed. Sensitive electronic trasducers enable precise measurements of pressure changes and movement. They have also found new translucent or transparent media to examine

Question: What does priapulid locomotion and Wendy’s fast food chain have in common?

Answer: They both use methyl cellulose. This is the main ingredient in a Wendy’s Frosty and is also used as a transparent burrowing medium for functional morphology studies of invertebrate burrowing behavior.

The density of methyl cellulose and marine sediment are comparable.

There are two general means of burrowing in soft sediment for most inverts.

One is peristalsis with a penetration anchor and terminal anchor.

Priapulids

The other is with an eversion and retraction of a body part such as a proboscis, pharynx, or introvert.

Here the trunk forms the penetration anchor, and the introvert is trust forward. Then the introvert is swollen and then serves as a terminal anchor to drag the rest of the animal behind. Polychaete annelids, kinorhyncs and according to Reinhardt Kristensen, probably some loriciferans.

Explain movement patterns and importance of introvert.

Energetic studies of burrowing have found it to be the most energetically costly form of locomotion.

SHOW OVERHEAD COMPARING RUNNING, SWIMMING, FLYING AND BURROWING

With larger size, comes reduced burrowing costs in soft oozy marine sediment. In sandy sediment, the opposite may be true (such as where some kinorhyncs live). In either case, there must be great benefits to living in such an environment to deal with high locomotory costs.

They produce a loricate larvae that, like the adult has a retractable introvert.

SHOW OVERHEAD PAGE 357 BRUSCA?

Suming up the pseudocoelomates:

SHOW OVERHEAD PAGE 227 INVERTEBRATE RELATIONS

How are all these animals related to each other?

There is alot of convergent evolution-consider the infaunal or interstitial habitats of some of these burrowing animals. Animals that are small tend to have similar losses of respiratory and circulatory structures. Also interstitial animals tend to have scales, spines or similar structures in common.

Several of the phyla have an introvert or retractable proboscis. Unfortunately trying to figure out their relationships has been a mess. Molecular data will probably be necessary to clarify relationships.

It is likely that not all of these phyla are monophyletic, and of those that are, they probably diverged a very long time ago and have been evolving in isolation from each other for some time.

The arguments chiefly revolve around whether the pseudocoelom evolved once or more than once.

SHOW PSEUDOCOELOMATE TREE

Here is one evolutionary tree from last year based on morphological characters

The paper concludes by suggesting that the pseudocoel is of little phylogenetic value because pseudocoelomates are probably polyphyletic. Molecular data-which I’ll talk about later, seems to agree with this conclusion.

This phylogenetic analysis suggests that the pseudocoel evolved twice or that it evolved once and was independently lost in the Gastrotricha and Gnathostomulida.

SHOW CHARACTERS OF PSEUDOCOELOMATES

Wallace, Ricci, and Melone (1996 Invertebrate Zoology. 115(2): 104-112)

Exhibited in Rotifers (later you'll see that some crustaceans exhibit it too). Change in morphology due to a chemical cue produced by a predator or other environmental factor.

1) Coelomate

2) Protostome, Schizocoelous, Spiral cleavage, determinate cell fate

3) Metamerism (serial segmentation)

4) Closed true circulatory system

5) Some have specialized respiratory structures

6) Complete gut, regionalized

7) Well developed nervous system, with ventral nerve cord, dorsal cerebral ganglion

8) Possess metanephridia (but some have protonephridia)

9) Separate sexes or hermaphroditic with trochophore larvae

10) Marine, freshwater, or terrestrial