SECTION VIII 

 PHYLUM MOLLUSCOIDA 1 



THE phylum Molluscoida comprises three classes the Polyzoa 

 (including, provisionally, the Endoprocta), the Bmchiopoda, 

 and the Phoronida. The members of these three classes are 

 tolerably widely divergent, so that it is somewhat difficult to 

 frame a general account of the entire phylum ; but the following 

 are the most important common features : 



There is, except in the Endoprocta, a body-cavity (coelome), lined 

 in most cases with a coelomic epithelium, within which the ali- 

 mentary canal is suspended by means of mesenteries or by means 

 of funicular strands taking their place. The dorsal region of the 

 body is abbreviated, being represented only by a short space 

 between the mouth and anus, which are close!} 7 approximated. 

 There is a lophophore or tentacle -bearing ridge, usually of a horse- 

 shoe shape, containing a special compartment of the coelome, and 

 overhanging the mouth on its anal side there is in most cases a 

 sensitive process the epistome also containing a special com- 

 partment of the body-cavity. The central part of the nervous 

 system consists of a single ganglion (supra-oesophageal), or of two 

 ganglia (supra-cesophageal and infra-cesophageal), or of a nerve- 

 ring. The nephridia when present are in nearly all cases a single 

 pair of ciliated tubes, which act also as gonoducts. 



CLASS I. POLYZOA. 



The Polyzoa form colonies known as " Sea-mats," or " Coral- 

 lines," which in many cases bear a close general resemblance to 



1 This and all the remaining phyla of the animal kingdom are characterised 

 by the possession of a true coelome, i.e. of a cavity interposed between the 

 wall of the body and that of the enteron, and developed either directly by 

 outgrowth from the archenteron, or formed from clefts that appear in solid 

 masses of mesoderm cells. The only group hitherto dealt with in which a definite 

 coelome is present is the Chsetogiiatha. In some of the groups which are here 

 comprised in the ccelomate phyla, however, as will be seen, the coelome is 

 reduced, or entirely absent, or not typically developed. 



340 



