CALYPTOBLASTEA 135 



The hydranth of Obelia is an expansion of the coenosarc, ending in 



a prominent oral cone, surrounded by a single ring of rather numerous 



tentacles, which have a solid core of endoderm cells. The blastostyle 



has a mouth but no tentacles; the body wall proliferates to form 



distinct individuals, the medusae. Those nearest the mouth of the 



gonotheca mature first, and they are 



liberated as they mature. The medusa 



of Obelia (Fig. 113), the type of the 



Leptomedusae, is like a shallow saucer, -^^^^^i?^ w ■■■;^^^.. 



the middle of the concave (subumbrel- "^ 



lar) surface being produced into a short 



manubrium. The rim of the medusa ^^| /A?^X.i i^^-^ew. 



bell is furnished with a large number 



of short tentacles. Like all medusae 



belonging to the Hydromedusae, it 



has four radial canals, running from 



the gastric cavity to the circular canal . ^^^- ^ ^3 • Free-swimming me- 



On the course of the radial canals and, ^J!;^j\/ro^T> -jI^^' ^ ^°^- i ^^ T 



' and MacBnde. caw.r. radial canal; 

 at the end of a short branch, patches ^. gonad; M. mouth at end 

 of the subumbrellar ectoderm are of manubrium; ot. otocyst; ten. 

 modified to form the gonads. The tentacles. 



germ mother-cells originate in the ectoderm of the manubrium, 

 pass through the endoderm and along the radial canals to the 

 gonads and then migrate into the ectoderm again. Only male or 

 female germ cells are produced by each medusa. At regular 

 intervals in the circumference are eight sense organs, the statocysts. 

 They are tiny closed vesicles, lined with ectoderm and filled 

 with fluid in which minute calcareous grains occur. The epi- 

 thelial lining not only secretes these but is also sensory: the impact 

 of the grains on the cells produces a stimulus which is transmitted 

 through the nerves to the muscles, and if the position of the 

 medusa should be abnormal the muscles contract in such a way as 

 to right the bell of the animal. 



Another characteristic of the hydrozoan medusa is the velum 

 (which is practically absent in Obelia)^ a narrow internal shelf running 

 inside the border of the subumbrellar cavity. This is largely composed 

 of ectodermal circular muscles, separated by a horizontal partition of 

 structureless lamella. At its base is a double nerve ring: the inner 

 half of this is concerned with the subumbrellar musculature and the 

 outer with the sense organs. 



The ripe ova are shed into the water by the rupture of the gonad, 

 and fertilization takes place here. Segmentation leads to the formation 

 first, of a hollow blastula, and from this, by the immigration of cells at 

 one pole, the elongated planula larva (Fig. no) with a solid core of 

 endoderm is formed. It is ciliated and swims freely for a time, eventu- 



