180 



Polyzoa. 



body-cavity, and at the other, to the exterior by a common aperture, 

 near the lophophore * The Polyzoa usually have a large body-cavity, 



filled with a liquid in which 

 amoeboid cells are found ; 

 it contains, besides the ali- 

 mentary canal, a cord, the 

 funiculus (Fig. 142), 

 stretching from the stomach 

 to the body-wall, upon which, 

 or upon the inner side of the 

 body-wall, ova and sperma- 

 tozoa appear, both, usually, 

 in the same individual ; special 

 sexual ducts are absent, the 

 genital products (or embryos) 

 pass out through holes in the 

 body-wall, or through the 

 excretory organs. G-enerally, 

 the fertilised ovum undergoes 

 its earliest development within 

 the body of the parent, in 

 many marine forms, in a 

 special invagination of the 

 body-wall (ooecium). 



Among the freshwater Poly- 

 zoa reproduction is effected by 

 means of statoblasts, as 

 well as by fertilised ova. The 

 statoblasts are small, discoid 

 bodies arising upon the funiculus by a peculiar process of budding. They are 

 produced chiefly towards the end of the, summer, and rest during the winter, 

 developing, in the next year, into a new colony. Each is provided with a hard 

 ornamental shell, in whose edge there are small air cavities. The new animal is 

 formed from a mass of cells within. 



In many forms a very remarkable degeneration of the lophophore and 

 alimentary canal occurs, constituting the so-called " brown body," from which 

 these parts are, after a time, reconstructed. 



The colonies formed by the Polyzoa are of very different 

 kinds. Some are much branched (Fig. 141), and either stand erect 

 from, or creep over, some foreign object; others are laminate, lying 

 upon the substratum or standing upright : or they may be more 

 massive. The colony is almost always fixed; a single freshwater form 

 {Cristatella) is free. 



Amongst many of the Chilostoma, dimorphism, like that in the 

 Hydrozoa, occurs. Specially common among the ordinary individuals 



Fig. 141. Plumatella pohimorpha, a fresh- 

 water Polyzoon. Enlarged. — After Krapelin. 



* These canals do not seem to form excretory products themselves, but serve as 

 a means of exit for cells, loosened from the epithelium of the body-cavity, in whose 

 protoplasm certain nitrogenous waste products are secreted. 



