( ■(] I l.\ I 'KM ATA 



65 



most part colorless, and constitute what are commonly called 

 the small jell\ fishes (Fig. 48). They are more or less bell- 

 shaped, so that we speak of the convex surface as the exum- 

 brella, and the concave as the subumbrella. The margin of 

 the bell is smooth and has attached to it tentacles, and also a 

 delicate membrane, which extends inward like a circular shelf 

 around the inner edge; this is known as the velum, or craspr 

 don. Organs of sight and hearing, known respectively as eyes 

 and otoevsts, are also found, freely exposed, at the margin of 

 the bell. Suspended in the 

 centre of the subumbrella 

 is an elongated structure, 

 the manubrium, at whose 

 free end is the mouth. The 

 mouth leads into the coel- 

 enteron, which consists of 

 the cavity of the manu- 

 brium opening into a cen- 

 tral cavity, from which four 

 radial canals pass to the 

 margin of the bell, where 

 they connect with a circu- 

 lar canal. In some cases 

 there are more than four 

 radial canals. The coelen- 

 teron is lined throughout 

 with entoderm, the exterior 

 of the animal is covered 



with ectoderm, and between the two is a basement membrane, 

 or a mesenchyme. The germ-cells develop from ectodermal 

 cells lying on the radial canals in the subumbrella. There is 

 often an alternation of generations between the polyps and the 

 medusae, polyps giving rise to medusas and vice versa, and in 

 some cases free-swimming colonies are produced, which consist 

 of both kinds of individuals. 



FlG. 48. Oceania laiiguida, a craspedote medusa, 

 highly magnified. (Drawn from life by the author.) 



Order 1. Hydridae 



The Hydridae (Gr. vSpa, hydra) constitute a very small order 

 of fresh-water polyps, some brown and some green. They have 



