156 THE INVERTEBRATA 



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. 115), the 



type of the Leptomedusae, is like a 



shallow saucer, the middle of the 



concave (subumbrellar) surface being 



produced into a short manubrium. 



The rim of the medusa bell is '^^l^ /^3^^ ? I^S-^en. 



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. Fig. 115. Free-swimming me- 



On the course ofthe radial canals and, dusa of Ohelia. From Shipley 



at the end of a short branch, patches ^^^ MacBride ca«.r. radial canal; 

 r . 1 1 11 ^ J R- gonad: M. mouth at end 



of the subumbrellar ectoderm are ^^ j^anubrium; 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, in 

 the Trachylina only, 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. 112) with a solid core of 

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

 ally settling down by its broader end, while the other end develops 

 a mouth and tentacles surrounding it. The endoderm delaminates to 



