IS 



INVEllTEBEATE ANIMALS. 



each consisting of a number of distinct but similar zobids, produced 

 by gemination from a single primordial individual. The colonies 

 thus produced are very generally protected by a horny or chitinous 

 integument, and they are so like the Hydroid Zoophytes that they 

 were long described as such. Indeed, the only absolute distinction 

 between the two classes is to be found in the internal structure of 

 the zooids of each. The separate beings or zooids which collec- 

 tively constitute the colony of any Polyzoon are spoken of as " poly- 

 pides " — the term polypite being only used in connection with the 

 Hydrozoa, and the term pulype being similarly restricted to the 

 Artinnzon. 



Each polypide in a typical Pulyzoun has the following structure 

 (fig. 1.32, 2,) : The body of the animal is enclosed in a double- walled 

 sac, of which the outer layer is usually chitinous or calcareous, and 

 constitutes a " cell " in which the zooid is contained. This outer 

 layer is known as the " ectocyst " to distinguish it from the ecto- 



Fig. 132. — ^Inrpliolngy of Polyzoa. 1. Fragment of one of tlie Sea-mats (Flusira 

 tntiti^ata) ina;.,'iii(ied to show the cells. 2, Diagram of a single polypide of a Poly- 

 20011 (after AUman) : a ]\Iouth surrounded by the ciliated tentacles ; & Alimentary 

 canal; c Anus; il Nerve-ganglion: c Investing sac or "ectocyst"; // Reproduc- 

 tive organs; f/ Muscle. 3. Bird's-head process. 



derm of the Ccdenterata. The cell, thus formed, is lined by a much 

 more delicate membranous layer, which is known as the "endooyst." 

 This membranous sac, formed by the endocyst, is pierced by two 

 openings. One of these is the mouth, and it is always surrounded 

 by a circle or crescent of hollow ciliated processes or tentacles (fig. 



