254 INVERTEBRATE MORPHOLOGY. 



CHAPTEE XL 



TYPE PPvGSOPYGIA. 



THE members of the type Prosopygia are compact, soli- 

 tary, or colonial organisms destitute of a true metamerism 

 and having the digestive tract usually bent upon itself, so 

 that the anus lies in more or less close proximity to the 

 mouth and therefore near the anterior end of the body. A 

 chitinous or more or less calcareous investment is formed 

 about the exterior of the body, and in some cases assumes 

 the form of a calcareous bivalve shell, similar to that of the 

 Pelecypoda in its general appearance, although in the rela- 

 tions of the valves to the body-surfaces and in other particu- 

 lars (see p. 327) there are very decided differences, the simi- 

 larity being simply an analogy. 



A more characteristic feature, however, is the presence at 

 the anterior end of the body of a circular or horseshoe-shaped 

 fold, or else of two armlike lateral processes, forming what 

 is termed the lophophore, upon which are borne a number of 

 tentacles which play important roles not only in obtaining 

 food, but also in the process of respiration, no branchial or 

 other special respiratory organs being present. 



A more or less spacious coeloni is usually present, trav- 

 ersed by muscle-fibres and some specially developed muscle- 

 bauds, though the muscular system is on the whole poorly 

 developed. The ccelom contains a hsemolymph, but a sepa- 

 rate blood vascular system and heart is entirely wanting. 

 The nervous system, in accordance with the absence of met- 

 amerism, is exceedingly simple, consisting either of a single 

 ganglion, lying between the mouth and anus and sending off 

 nerves to the various regions of the body, or else of a nerve- 

 ring surrounding the oesophagus, with more or less distinct 

 supra- and sub-cesophageal ganglionic enlargements. Special 

 sense-organs are wanting. 



