PHYSIOLOGY OF THE INVERTEBRATA. 447 



The Polyzoa. 



Reproduction in these animals takes place by gemmation, 

 parthenogenesis, and gamogenesis. Where gemmation occurs, 

 the buds produced usually remain adherent to the stock ; but 

 in Pedicellina and Zoxosoma, they become separated. Gem- 

 mation occurs throughout the whole colony of polypides. In 

 the fresh water Polyzoa, a kind of parthenogenesis is the mode 

 of reproduction. In these animals, gemmules, statoblasts, or 

 unfecundated ova, are developed in the funiculus. A stato- 

 blast is usually biconvex in form, covered by two chitinous 

 shells, and gives rise to an animal which, when hatched, 

 resembles the adult. It soon becomes fixed and produces 

 (by gemmation) a new colony of organisms. Sexual repro- 

 duction or gamogenesis always takes place in the Polyzoa. 

 As a rule they are usually hermaphrodites. The male and 

 female organs are groups of cells, the former being developed 

 either in the upper portion of the funiculus or at its base ; 

 and the latter on the internal surface of the anterior part of 

 the endocyst. The ova fall into the perivisceral cavity, where 

 they are fecundated by the spermatozoa, which are also 

 present in the same cavity. The fecundated ovum (in marine 

 Polyzoa^ passes into the ovicell — a dilatation of the body wall. 

 The ovum becomes segmented in the ovicell, forming a 

 morula, and subsequently a blastula. Finally, the embryo is 

 hatched as a ciliated free swimming larva (Trochosphere). 

 After swimming about for some time, the larva becomes 

 stationary and develops a tentacular crown and "cell." 

 New zooids are then produced by gemmation, and so a new 

 •colony is gradually formed. 



The Brachiopoda. 



Most of these animals are dioecious ; and the genital organs 

 are situated in the body cavity or its prolongations. They 

 consist of paired, glandular bands ; and the spermatozoa and 



