116 OUTLINES OF EVOLUTIONARY BIOLOGY 



the hypostome (Fig. 57, B), which lies within the circle of 

 tentacles. 



The endoderm, which immediately lines the gastral cavity, is 

 made up of a single layer of relatively large cells (Fig. 58, I) 

 whose function is digestive. The ectoderm is made up of several 

 kinds of cells, some larger than others. The larger ones (A) 

 are much broader at their outer than at their inner ends 

 and the interstices thus left between the latter are filled up 

 by small interstitial cells (G). The endoderm cells and the 

 larger ectoderm cells both send out prolongations of their bodies 

 into the gelatinous mesogloea (H) which lies between them, and 

 these processes, having the form of elongated fibres (C), are the 

 seat of that power of contraction which Hydra possesses in such 

 a high degree they are in fact muscular. 



Hydra has two very distinct methods of reproduction, asexual 

 and sexual respectively. The former consists in a process of 

 budding, little hollow outgrowths of the body being formed, 

 which elongate, acquire mouth and tentacles, and for a time 

 remain attached to the parent (Fig. 57, K). In this way tem- 

 porary colonies of polypes may be produced, but sooner or later 

 the buds separate and begin to lead independent lives. 



Sexual reproduction is effected by means of ova and sperma- 

 tozoa, which are essentially similar to those of higher animals. 

 They are produced in gonads ovaries and testes and as 

 both kinds of gonad usually occur in the same individual the 

 animal is hermaphrodite. The testes (Fig. 57, H) take the form 

 of little swellings situated at a short distance beneath the ring 

 of tentacles and formed each by an accumulation of interstitial 

 ectoderm cells. These are the primordial germ cells, by the 

 division of which the spermatozoa are formed. The spermatozoon 

 is perfectly typical, resembling a flagellate protozoon, with an 

 ovoid head consisting almost entirely of chromatin and a long 

 cytoplasmic tail or flagellum by means of which it swims 

 actively about when shed into the surrounding water by rupture 

 of the testis. 



There is usually only a single ovary, appearing as a larger 

 projection from the body wall nearer to the attached end of 

 the animal ; it consists at first, like the testes, of a heap of 

 primordial germ cells formed by the multiplication of interstitial 

 cells. In each ovary, however, only a single cell develops into a 

 mature ovum. (Fig. 57, I), its sister cells, which may all be 



