HYDRA. 



311 



Fig. 38. 



between its investing membranes the ovum is developed 

 (Woodcut, fig. 38). After a time it bursts the ectodermal 

 covering which confines it, and 

 remains attached by a kind of 

 pedicel. At this stage a very 

 strong, elastic shell or capsule 

 forms round the ovum, the sur- 

 face of which is, in some cases, 

 studded with spine-like points, in 

 others tuberculated, the divisions 

 between the tubercles being polygo- 

 nal*. After a time the ovum drops 

 from its pedicel and becomes at- 

 tached by means of some mucous 



secretion, in which state it remains until the liberation of 

 the embryo. The young Hydra, on issuing from the egg, 

 has four rudimentary tentacles (Woodcut, 

 fig. 39) . The ova are occasionally produced 

 in spring, and in this case they are hatched 

 in the course of the summer; but more 

 usually they are developed late in the 

 autumn, when gemmation has quite ceased, 

 and undergo no change till the following 

 year. The spermaries are developed as small 

 conical projections, a little below the base of the tentacles 

 (Woodcut, Fig. 40, a) . 



The Hydra may also be multiplied indefinitely by me- 

 chanical division ; almost every portion, separated from the 

 rest, is capable of producing a perfect polypite. It has 

 even been observed to break up of itself into numerous 

 particles, which lived on for a considerable time, and at 



* Vide a paper ' On the coexistence of ovigerous and spermatic capsules 

 on the same individuals of the Hf/clra viridi?'' by Prof. Allen Thomson 

 (Proe. Roy. Soc. Edinb. No. :J<>. 1*17 >. 



Fig. 39. 



