340 ZOOLOGY SECT. 



tentacles, and alimentary canal having been 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, ciliated, 

 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 tentacles are capable of being withdrawn.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 the 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 Endoprocta. 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 coelome. 



ORDER 1. GYMNOLJEMATA. 



Almost exclusively marine Ectoprocta, with a circular lopho- 

 phore, and without an epistome. 



Sub-order a. Gydostomata. 



Gymnolsemata with tubular calcareous zorecia having circular 

 apertures devoid of closing apparatus. 

 Including Crisia, Idmonea, &c. 



Sub-order b. Cheilostomata. 



Gymnolaemata with calcareous or chitinous zooecia usually pro- 

 vided with opercula. 



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

 Selenaria. 



Sub-order c. Ctenostomata. 



Gymnolaemata with chitinous or gelatinous zocecia provided with 

 a series of tooth-like processes closing the aperture when the 

 tentacles are retracted. 



Including Alcyonidium, Serialaria, Paludicella. 



