T7PE PR0S0P7O1A. 255 



A pair of simply-constructed nephridia are present in 

 some forms, but in many a special excretory organ seems to 

 be entirely wanting. Bisexuality is the usual arrangement, 

 although in the Polyzoa hermaphroditism is of not unfre- 

 quent occurrence. 



The great majority of the Prosopygia are marine in habi- 

 tat, though a number of Polyzoa are inhabitants of fresh 

 water. The type may be divided into two well-marked 

 classes, the Polyzoa and the Brachiopoda. 



I. Class Polyzoa. 



The Polyzoa, a group usually spoken of by German zool- 

 ogists as the Bryozoa, are almost without exception colonial 

 organisms, forming encrusting, massive, or more or less den- 

 dritic masses composed of a large number of small individuals 

 or polypides, each of which is enclosed within a chitinous or 

 in some cases partially calcified investment, the zooecium, from 

 the mouth of which the anterior portion of the body bearing 

 the lophophore may be protruded. This outer investment or 

 ectocyst (Pigs. 115 and 116, ec) is lined upon its interior sur- 

 face by a layer of ectoderm-cells, within which is a delicate 

 peritoneal lining, these two layers together constituting the 

 true .body-wall or endocyst (Fig. 115, en) practically destitute 

 of muscle-tissue, though a sphincter is usually present at 

 the mouth of the cup, which may thus be closed over the re- 

 tracted polypide. 



A more or less spacious ccelom (Pig. 115, co) is present in 

 the majority of forms, containing a hsemolymph and tra- 

 versed by a number of muscle-strands (m) which may be 

 aggregated into special retractor bands ; but in one order, the 

 Endoprocta (Pig. 114), these are wanting and indeed the 

 coelom is reduced to a very small space between the body- 

 wall and the digestive tract. This latter structure has the 

 characteristic U- or Y-shaped form and presents but little 

 differentiation into special parts, though an oesophagus, 

 stomach, lined with glandular so-called liver-cells, and rectum 

 may be distinguished. An anus (Pigs. 115 and 116, a) is 

 always present and may be situated either within or without 



