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MOLLUSCOIDEA. 



POLYZOA. 



Class I. Polyzoa (Bryozoa). — The members of this class are 

 mostly composite animals, each zooid of which possesses, typically, a 

 freely suspended alimentary canal, with mouth and anus, enclosed with- 

 in a double-walled sac. The mouth is surrounded with a circle or 

 crescent of hollow ciliated tentacles, and the nervous system consists of 

 a single ganglion placed between the mouth and the anus. 



With the single exception of the genus Loxosoma, all the Polyzoa 

 live in an associated form in colonies or " polyzoaria," which are 



sometimes foliaceous, sometimes branch- 

 ed and plant-like, sometimes laminar, 

 sometimes encrusting, and very rarely 

 are free. Each " polyzoarium " consists 

 of an assemblage of distinct but similar 

 zooids arising by continuous gemmation 

 from a single primordial individual. 

 The entire colony — or its entire der- 

 mal system — is called the "polyzoar- 

 ium " or " ccencecium " ; the separate 

 zooids are called " polypides " ; and 

 the little chambers in which each is 

 contained are called the "cells," or 

 "zocecia." 



A typical polypide of a Polyzoon (fig. 

 450) consists of a membranous sac (" en- 

 docyst ") the external surface of which, ex- 

 cept at its anterior end, is generally hard- 

 ened by an investing horny or calcified 

 layer, which is known as the " ectocyst." 

 Freely suspended in the perivisceral 

 fluid filling the space enclosed with- 

 in the endocyst is the alimentary canal, 

 which consists of a mouth, gullet, stom- 

 ach, intestine, and anus. The mouth 

 (fig. 450, m) is placed anteriorly, and is 

 surrounded by a crown of tubular, non- 

 retractile tentacles, which are ciliated, 

 and act as respiratory organs. In differ- 

 ent types of the class, the tentacles are 

 arranged in a circle or are disposed so 

 as to form a horse-shoe or crescent. 

 The alimentary canal is bent upon it- 

 self in such a way that the anus (fig. 450, a) comes to be placed 

 near the mouth, being usually placed outside the tentacular circle 



Fig. 450. — Diagram showing the 

 structure of a single polypide of a 

 Polyzoan (after Busk). /, Tentacles ; 

 m, Mouth ; g, Nerve-ganglion ; ce, 

 Gullet ; s, Stomach ; i, Intestine ; 

 a, Anus ; o, Ovary ; x, Testis ; z, 

 Funiculus ; or, Aperture of the 

 zocecium ; v, Tentacular sheath ; 

 d, Perivisceral cavity ; r, Retractor 

 muscles; c, Ectocyst. 



