CHAPTER XX. 



MOLLUSCOIDA. 



CLASS I. POLYZOA (Gr. polus, many ; zoon, animal.) 

 The members of this class are the lowest of all Molhtsca, 

 and they are generally known by the popular names of " Sea- 

 mosses " and " Sea-mats." They are invariably compound, 

 forming associated growths or colonies, each consisting of a 

 number of distinct but similar zoftids, produced by gemmation 

 from a single primordial individual. The colonies thus pro- 

 duced 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. The only absolute distinc- 

 tion between the two classes is to be found in the internal 

 structure of the zoo'ids of each ; . but they may be generally 

 separated by the fact that the separate cells in a compound 

 Hydroid are all united to one another by means of a common 

 flesh or ccenosarc ; whereas in the Polyzoa the separate cells 

 composing the colony are merely connected externally, but 

 very rarely have any direct communication with each other. 

 The separate beings or zooids which collectively constitute the 

 colony of any Polyzoon are spoken of as " polypides " the 

 term polypite being only used in connection with the Hydro- 

 zoa, and the term polype being similarly restricted to the 

 Actinozoa. 



Each polypide in a typical Polyzoon has the following 

 structure (Fig. 76, 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 zo5id is 

 contained. This outer layer is known as the " ectocyst," to 

 distinguish it from the ectoderm of the Ccelenterata. The 

 cell, thus formed, is lined by a much more delicate membra- 

 nous layer, which is known as the " endocyst." This membra- 



