328 



ZOOLOGY 



SECT. 



The Endoprocta are only doubtfully to be included in the same 

 class with the Ectoprocta. The position of the anus, the absence 

 of the introvert, and the presence of typical nephridia would not in 

 themselves be sufficient to justify their removal from the Polyzoa, 

 and perhaps not even the obliteration of the coelome. But should 

 certain statements, which have been published with regard to 



their development and metamorphosis, be con- 

 firmed, we should be driven to the conclusion 

 that the Endoprocta are not directly related 

 to the Ectoprocta at all. The gist of these 

 statements is that the line joining mouth and 

 anus in the Endoprocta is ventral and not 

 dorsal, the ganglion infra-cesophageal, and the 

 tentacular circlet prse-oral ; and if this should 

 be established the structures named cannot be 

 homologous in the two groups. 



CLASS II. PHORONIDA. 



The position ofPhoronis a worm-like marine 

 animal is a matter of uncertainty ; but it ex- 

 hibits some unmistakable points of resemblance 

 to the Polyzoa, more particularly to the Phy- 

 lactoloeinata, and it may very fairly be dealt 

 with as a third class of the Molluscoida. 

 _ Phoronis (Fig. 261) lives in associations con- 

 sisting of a number of individuals, all of which 

 are developed from ova, there being no process 

 of asexual formation of buds. Each worm is 

 enclosed in a membranaceous or leathery tube, 

 within which it is capable of being completely 

 retracted. The body is cylindrical, elongated, 

 and unsegmented. At one end there is a 

 crown of numerous slender ciliated tentacles 

 borne on a horse-shoe-shaped lophophore, the 

 lateral cornua of which are spirally coiled in 

 the larger species ; these are supported by a 

 mesodermal skeleton and are non-retractile. 

 Both mouth and anus (Fig. 262, mo, an} are 

 situated at this tentacular extremity of the 

 body, separated from one another by only a short space. A small 

 lobe the epistome (ep) overhangs the mouth and lies between 

 it and the anus. Near the anus open two ciliated nephridial 

 tubes (nepli) of mesodermal origin, which open internally into the 

 posterior chamber of the body-cavity. The ccelome is lined with 

 a peritoneum from which there proceed three mesenteries (Fig. 263) 

 a ventral longitudinal and two transverse, the latter dividing 



FIG. 261. Phoronis 

 australis, natural 

 size. 



