MOLLUSCA: POLYZOA. 379 



B. ENTOPROCTA. 



Mouth and anus both within the circle of tentacles ; lophophore horse- 

 shoe-shaped. Tentacles solid and non-retractile, filled, like the body- 

 cavity, with parenchyma. Ectocyst not calcified. This division includes 

 the marine Pedicellina and Loxosoma, and the fresh-water Urnatella. 

 Loxosoma is semiparasitic, and is attached to the bodies of Gephyreans 

 and other marine animals. 



C. ASPIDOPHORA. 



This division includes only the singular marine genus Rhabdopleura, in 

 which the lophophore is crescentic, and carries a discontinuous series of 

 tentacles ; the mouth is lateral rather than terminal ; a special shield-like 

 organ is attached to the body of the lophophore, between the mouth and 

 the anus ; the coenoecium is chitinous and tubular, and is supported by a 

 correspondingly divided chitinous rod, attached superiorly to a fleshy con- 

 tractile cord, which is in turn connected with the body of the polypites ; 

 and, lastly, the endocyst and tentacular sheath are wanting. 



Of the above divisions of the Polyzoa, the two most important groups 

 are those of the Phylactolczmata and Gymnolcemata. In the former of 

 these are included almost all the fresh-water Polyzoa, and the lophophore 

 is bilateral and horse-shoe-shaped in all except Fredericella. The division 

 of the Gymnolamata, on the other hand, includes the fresh-water genera 

 Paludicella and Urnatella, and the vast majority of the marine Polyzoa. 

 Of these latter, the sub-order of the Cheilostomata is the most important, 

 as embracing the greater number of the common forms.- In these, the 

 opening of the cell is sub-terminal, and is generally closed by a movable 

 lip or shutter. On the other hand, in the sub-order Cydostomata, the cells 

 are tubular, the orifices terminal, of the same diameter as the cell itself, 

 and without any movable apparatus for closure. Lastly, in the singular 

 group of the Ctenostomata (including Vesicularia, Alcyonidium, and Val~ 

 keria\ the cells arise from a common tube, and their mouths are terminal, 

 and furnished with a setose fringe for their closure. 



AFFINITIES OF THE POLYZOA. -By many zoologists the Poly- 

 zoa are now regarded as being an anomalous class of Worms, 

 closely related to the true Annelides. That there are points of 

 relationship between these apparently diverse groups cannot 

 be doubted ; but these do not seem sufficient to outweigh the 

 points of divergence such, for example, as the absence of seg- 

 mentation in the former, and the totally different form of the 

 nervous system. The Polyzoa have also striking affinities with 

 the Brachiopoda and Tunicata, and even with some of the Mol- 

 lusca proper ; and it has not, therefore, appeared advisable to 

 remove them to the division of the Anarthropoda. 



DISTRIBUTION OF POLYZOA IN SPACE. The Polyzoa, like all 

 the Molluscoida, are exclusively aquatic in their habits, but 

 unlike the remaining two classes, they are not exclusively con- 

 fined to the sea. The marine Polyzoa are of almost universal 

 occurrence in all seas. The fresh-water Polyzoa, however, not 

 only differ materially from their marine brethren in structure, 



