SECTION VIII 

 PHYLUM MOLLUSCOIDA 



THE Phylum Molluscoida comprises the three classes of the 

 Polyzoa (including, provisionally, the Endoprocta), the Brachiopoda, 

 and the PhoronidcL 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 true body-cavity, lined in 

 most cases with a coelomic epithelium, within which the alimen- 

 tary 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 closely approximated. 

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

 shoe shape, containing 1 a special compartment of the coelome, and 

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

 sensitive process the ep'istomc-* also containing a special com- 

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

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

 ganglia ( supra-oesophageal and infra-oesophageal), 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 

 Hydroid Zoophytes, and are only on a more minute inspection 

 found to differ totally from the latter, and to exhibit a very much 

 higher type of structure. 



