230 Elementary Biology. 



sions made upon them by the external environment, might 

 naturally be expected to be ectodermal in origin, and the 

 nerve-cells of the medusoid have been found to originate 

 from superficial cells which have become sunk in the more 

 or less gelatinous tissue that fills the spaces between the 

 ectoderm and endoderm. This gelatinous tissue, through 

 which numerous cells are scattered, is looked upon as a 

 middle layer, much more highly differentiated in the higher 

 animals, and termed there the mesodenn. 



Moreover in the free-swimming gonophore we meet for 

 the first time with sense-organs, special nervous organs for 

 the reception of impressions of the outside world, whether 

 in the form of vibrations of light or of sound. In the 

 neighbourhood of these organs the nervous elements are 

 very fully developed ; indeed, the cells which make up the 

 sense-organ are in direct continuation with the terminations 

 of the nerve-cells. The sense-organs are of four different 

 kinds, ocelli or eyespots, otocysts and tentaculocysts or 

 organs of hearing, and olfactory pits or nasal organs, all of 

 which are found round the margin of the bell, and most of 

 them near the bases of the marginal tentacles. Since, how- 

 ever, these organs are specialised in their character we need 

 not enter into further detail with regard to them. 



Just as the thallus of the fern was an independent gene- 

 ration for the dissemination of the type, producing sexual 

 organs at a distance from the parent asexual plant, so the 

 medusoid is a very effective means for the dispersal of the 

 embryos of the Hydrozoa. After a certain period of free 

 existence the gonophore develops ovaria or spermaria be- 

 tween the ectoderm and endoderm layers in the manubrium 

 or on the course of the radial canals. 1 The sexual products 

 escape by the mouth or by rupture of the wall of the canal, 

 fertilisation taking place in the sea-water. The ovum is a 



1 Weisman has shown that the ova and sperms are really produced 

 in the stem and afterwards migrate into the gonophore. 



