190 MOLLUSCA. 



fishes, did not make their appearance till the commence- 

 ment of the Secondary period, and seem to have reached 

 their maximum in existing seas. 



The sub-kingdom Mollusca is divided into two great divi- 

 sions, termed respectively the Molluscoida and the Mollusca 

 Proper. The division Molluscoida comprises the three classes 

 of the Polyzoa, Tunicata, and Brachiopoda, characterised by 

 having a nervous system consisting of a single ganglion or prin- 

 cipal pair of ganglia, whilst there is either no distinct organ of 

 the circulation or an imperfect heart. In the division of the 

 Mollusca Proper are comprised the classes of the Lamelli- 

 branchiata (Bivalves), Gasteropoda (Univalves), Pteropoda, and 

 Cephalopoda. All these classes are distinguished by having 

 a nervous system composed of three principal pairs of ganglia; 

 whilst there is a well-developed heart, consisting of at least two 

 chambers. 



CLASS POLYZOA OR BRYOZOA. 



Animal composite, forming colonies, all the members of which 

 are produced by budding from a primitive being (zooid). Each 

 member of the colony (zooid) is enclosed in a double-walled sac, 

 the outer coat of which is mostly hardened by horny or calcareous 

 matter. There is no heart, and the mouth is surrounded by a 

 circle or crescent of hollow ciliated tentacles. The colonies are all 

 but invariably fixed to some foreign object, and are in many cases 

 plant-like inform. 



All the Polyzoa live in an associated form in colonies or 

 " polyzoaria," which are sometimes foliaceous, sometimes 

 branched and plant-like, sometimes encrusting, and very 

 rarely are free. Each " polyzoarium " consists of an assem- 

 blage of distinct but similiar zooids arising by continuous 

 gemmation from a single primordial individual. The colonies 

 thus produced are in very many respects closely similar to 

 those of many of the Hydroid Polypes, with which, indeed, 

 the Polyzoa were for a long time classed. The " polyzoarium," 

 however, of a Polyzoon differs from the polypidom of a com- 

 posite Hydroid in the general fact that the separate cells of 

 the former do not communicate with one another otherwise 

 than by the continuity of the external integument ; whereas 

 the zooids of the latter are united by an organic connecting 

 medium, or " ccenosarc," from which they take their origin. 

 On this point Mr Busk observes : 



" It has been before said that the Polyzoa are always asso- 

 ciated into compound growths, made up of a congeries of 

 individuals, which, though distinct, yet retain some degree of 



