UNSEGMENTED WORMS 203 



may retire into its own particular portion of the exoskeleton, when disturbed, by 

 the contraction of appropriate muscles. The brain consists of a single ganglion 

 lying between the mouth and anus. The two sexes usually occur in the same 

 individual. The colonies are formed by budding which takes place in each species 

 in a way that is characteristic of that species. Thus it comes about that the colo- 

 nies differ as much in general appearance as their individuals do in structure. 



Class II. BracTiiopoda (arm-footed; lamp-shells). Brachiopods are marine 

 forms chiefly of geological interest, as there are at present only a few living species. 

 They were very prevalent in early geological times. They possess a bivalved 

 shell which suggests that of the bivalve Mollusca (as the clam). From this 

 external resemblance they have long been classed as mollusks. The valves are 

 strictly dorsal and ventral in the Brachiopods, however; whereas in the mollusks 

 they are right and left. Their shell is therefore no longer considered as homologous 

 with the mollusk shell. The internal structure is still further removed from that 

 of the clam. The digestive tract is often bent much as in the Polyzoa, and the 

 mouth is surrounded by a tentacle-bearing lophophore (the "arms"). The 

 lophophore may have a skeletal support which in different types assumes different 

 shapes (loop, helix, or spiral). A peduncle usually extrudes at the hinge, by means 

 of which the animal attaches itself to foreign objects. The Brachiopods are not 

 colonial. The student is referred to the more extended texts for illustrations of 

 this group of animals. 



236. Some Forms of Doubtful Relationship. The old group "Vermes," or 

 worms, has in recent years had several fairly definite phyla removed from it. 

 Clearest among these is the phylum Annulata or segmented worms (Ch. XV). 

 More recently the Phyla Platyhelminthes, Nemathelminthes, Trochelminthes, 

 and Molluscoidea have been recognized by many students. In this old waste- 

 basket of worm-like animals are still the raw-materials for several more phyla 

 when we learn enough about them to distinguish them. None of these groups 

 is numerous in species, or of much economic or human interest. They cannot 

 receive much attention in an elementary course. Among them are: 



1. Mesozoa, which are simple, parasitic forms less complex even than the diplo- 

 Uastic animals. It ha? been thought that they are intermediate between protozoa 

 and metazoa. 



2. Nemertinea, similar in some ways to flat worms. They are peculiar in de- 

 velopment, and have some systems of organs better developed than other un- 

 segmented worms. They live in water, chiefly salt, and in moist earth. Some 

 attain a length of ninety feet. They are slender and usually flattened. 



3. Acanthocephala, spine-headed parasites sometimes classed with the nema- 

 todes. Pound in most vertebrates. 



4. Chatognatha, which includes Sagitta, the arrow- worm. Free-swimming 

 marine animals, sometimes placed near the segmented worms. 



5. Gephyrea, marine worms living in sand or mud, unsegmented with large 

 body cavity, and protrusible proboscis. The sipunculids are the best known 

 representatives. 



The student is asked to seek figures of some of these rarer types in the larger 

 texts, and notice the various ways in which they are classified. 



237. Notes on Ecology and Distribution. The organisim 

 included in this chapter represent the most varied modes of self. 



