PHYSIOLOGY OF THE INVERTEBRATA. 411 



mass which surrounds the body or somatic cavity differentiates 

 or divides into an ectoderm and endoderm. During all this 

 embryological development, the embryo has been growing in 

 length. At one end of the elongating embryo, a mouth is 

 formed by rupture of the tissues. "It first appears as a 

 star-shaped cleft which gradually becomes more or less round. 

 The tentacula next appear simultaneously. The animal then 

 bursts the thin membrane, comes out of it, and starts life on 

 its own account as a perfect Hydra. There is no meta- 

 morphosis in the development of Hydra (no invagination, 

 and no ciliated planula as in many other Hydroids). The 

 young Hydra passes into the adult condition by continuous 

 gro^vth." 



In the Mcduscc the sexes are separate ; the females have 

 yellowish-coloured ovaries, while the males possess rose- 

 coloured genital glands. The ova undergo their embryonic 

 development in the oval tentacula. The embryonic develo])- 

 ment of these animals presents the following phases : — 

 {a) egg ; (1)) morula (blastosphere) ; (c) gastrula (by invagi- 

 nation) ; ((/) planula (ciliated larva), this stage is formed by 

 the closing of the gastrula mouth and the "ciliating" of 

 the ectodermal cells ; (c) next appears the hydra-form or 

 scyphistoma, which is produced by the planula becoming fixed 

 and developing tentacula and a mouth at the free end. During 

 the scyphistoma stage there is at first multiplication by gem- 

 mation, but afterwards fission occurs, and the animal then 

 reaches the strobila stage; (/) the detached segments of the 

 strobila swim away in the ephyra form ; (cj) the ephyra 

 form after some weeks is converted into the adult animal 

 (in this case, Aurclia). In the development of Aurelia it 

 will be observed there is an alternation of generations ; the 

 asexual generation being represented by scyphistoma and 

 strobila. 



In the Actinia'- the sexes are united. The testes and 

 ovaries form closely convoluted tubules and the generative 

 products are discharged into the somatic or digestive cavity. 



