216 



ECTOPROCTA THROUGH ECHIUROIDEA: 



production resulting in a trochophore-like larva 

 which attaches to some solid substrate and develops 

 into the adult form. 



The lamp shells receive their common name be- 

 cause many resemble old Roman oil lamps. All liv- 

 ing species are solitary, usually attached, marine 

 animals. Most are shallow-water forms, but a few 

 are known from very deep water. They can some- 

 times be found in the lower tide levels under or on 

 protected areas of rocks. One living genus, Lingula, 

 has existed since the Ordovician period, some half- 

 billion years ago, and is probably the oldest living 

 genus of animal. 



worms (Annelida) and joint-legged animals (Arthro- 

 poda). 



The peanut worms, mollusks, echiuroids, seg- 

 mented worms, and joint-legged animals constitute 

 the schizocoelous animals without a lophophore. 

 These schizocoels can be further subdivided into 

 those without true segmentation (sipunculoids, mol- 

 lusks, and echiuroids) and those with true segmenta- 

 tion (annelids and arthropods). However, the peanut 

 worms and echiuroids are very closely related to the 

 annelids. Both taxa are sometimes classified as part 

 of the Phylum Annelida. In fact, all schizocoelous 

 phyla often are grouped in a Superphylum Annelida. 



UNSEGMENTED SCHIZOCOELA: 



SIPUNCULOIDEA, MOLLUSCA, 



AND ECHIUROIDEA 



It was mentioned previously that not all trocho- 

 phores are exactly the same. They vary between the 

 difiPerent phyla and in the individual species within 

 phyla. 



By now it should be obvious that the ancestry of 

 all phyla goes back into antiquity. In fact, there is 

 adequate fossil evidence for the statement that all 

 phyla had become phyla more than one-half billion 

 years ago! Therefore, when it is said that a certain 

 phylum is "closely related" to another, the statement 

 is not entirely accurate. If all phyla date back into 

 antiquity, present members of diflferent phyla cannot 

 be very closely related; hence, living protozoans and 

 chordates are, in a time sense, about as closely related 

 as are protozoans and sponges. What is meant then, 

 when one speaks of close relationship? The implica- 

 tion is simply that closely related phyla have retained 

 ancestral characters for eons, and only because of the 

 retention of characters are now similar. Of course, 

 this still allows for discussion of ancestry and re- 

 lationships, but with the time factor in mind, the 

 emphasis will be somewhat different. 



If we now turn to a particular line of development 

 within the trochophore-owners, the unsegmented 

 Schizocoela, we observe that many phyla diverged 

 from one common ancestral tendency. The first to 

 diverge were the peanut worms, followed shortly by 

 the mollusks. The echiuroids diverged next, but 

 even today show a great deal of similarity to the 

 segmented worms. Final segregation along this "line 

 of development" resulted in modern segmented 



SIPUNCULOIDEA (Peanut Worms) 



Diagnosis: symmetry bilateral; unsegmented; 

 schizocoelous; cylindrical; yellowish, grayish, or 

 brownish animals up to a foot long; superficially 

 resemble priapuloids or echiuroids, but anterior 

 retractile portion is terminated by tentacles, has fine 

 and chitinous papillae, and is not a true snout or 

 proboscis; anterior region withdraws inward as does 

 the finger of a glove; called "peanut worms" because 

 of the superficial, peanut-like appearance of the ani- 

 mals when their anterior portions are pulled into 

 their bodies; marine in sand, mud, empty shells, 

 crevices in rocks, etc. (Figure 13.4). 



Extended peanut worms resemble an elongated 

 flask. The opening of the "flask" contains short, 

 hollow, fringed, ciliated tentacles which are used 

 to capture microscopic food. The "flask neck" bears 

 fine, hard objects. When the animal contracts, the 

 head and neck are withdrawn into the body. The 

 body proper, the "bulb," is covered by a slightly 

 roughened cuticle. 



Figure 13.4 Various attitudes of Phasco/osomo, a peanut worm. 



