\m PHYLUM MOLLUSCOIDA 347 



described. In many of the zocecia of a fully-developed colony no 

 zooid is found to be present, but, instead, there is a dark brown 

 body similar to that which occurs in the primary zocecium. This 

 is a zooid that has undergone degeneration — the lophophore, 

 tentacles, and alimentary canal having become absorbed. Such 

 degenerated zooids are capable of regeneration, the organs becoming 

 re-developed and the brown body re-absorbed. 



2. Distinctive Characters and Classification. 



The Polyzoa are Molluscoida which, with one exception, form 

 colonies of zooids connected together by a common organic sub- 

 stance. There is a lophophore bearing a series of slender, cilated, 

 post-oral tentacles. The anterior part of the body forms, in the 

 majority, a short introvert, within which the lophophore and the 

 tentacles are capable of being withdrawn. In some the pro- 

 stomium is represented by a small lobe — jthe epistome. The 

 alimentary canal is U-shaped, and the anus is anterior, within, or 

 just outside, the tentacular circlet. In most the nervous system 

 is represented only by a small ganglion between the mouth and 

 the anus. A cuticle, sometimes gelatinous, sometimes horny, 

 sometimes calcified, forms a firm exoskeletal layer for the support 

 of the colony. Nephridia (corresponding to the head-nephridia of 

 the trochophore) occur only in the Endoproda. There is no 

 vascular system. The sexes are usually .united. The majority of 

 Polyzoa occur in the sea; a limited number are inhabitants of 

 fresh water. 



Sub-Class I. — Ectoprocta. 



Colonial Polyzoa with the anus outside the lophophore, with a 

 well-developed introvert and a spacious ccelome. 



Order ]. — Gymnolaemata. 



Almost exclusively marine Ectoprocta, with a circular lopho- 

 phore, and without an epistome. 



Sub- order a. — Cyclostomata. 



Gymnolaemata with tubular calcareous zooecia having circular 

 apertures devoid of closing apparatus. 

 Including Ci'isia, Idmonea, &c. 



Sub-order b. — Cheilostomata. 



Gymnolaemata with calcareous or chitinous zooecia usually pro- 

 vided with opercula. 



Including Bugula, Flustra (" Sea-mat ") Membranipora, Gellepora, 

 Selenaria. 



