1886.] 



MICROSCOPICAL JOURNAL. 



225 



point from which diverge the com- 

 pHcated nervous and muscular sys- 

 tems of higher animals. 



It remains to be seen whether or 

 not this certainly brilliant and attrac- 

 tive theory will stand the test of addi- 

 tional research, but it is impossible to 

 withhold admiration for its ingenuity, 

 or to deny its importance, especially 

 in its bearing upon the question of 

 nerve-terminations, and that of the 

 evolution of nerve and muscle. 



The curious nematocysts or thread- 

 cells (fig. 32), to which a benumbing 

 property has been ascribed by most 

 observers, originate in the interstitial 

 tissue, but as thev become mature 

 they are pushed towards the surface 

 of the ectoderm. Each consists of an 

 oval capsule, coiled within which is 

 a delicate spiral elastic filament, pro- 

 vided with 4 short recurved spines. 

 This can be suddenly unrolled with 

 considerable force, and by its projec- 

 tion into the body of the polype's 

 prey it is supposed that the latter is 

 frequently benumbed so that its cap- 

 ture by the tentacles is rendered 

 easier. 



At certain seasons of the vear the 

 reproductive organs of Hydra mav 

 be observed. They consist of sper- 

 matic capsules and of ovaria. The 

 first of these arise as conical whitish 

 elevations from the body wall, just 

 below the circle of tentacles (fig. 17, 

 s. c.) They are usually from 2 to 5 

 in number, but in some abnormal 

 cases as many as 20 have been ob- 

 served. In them are formed the 

 ciliated spermatozoa which are sub- 

 sequently ejected through the apex 

 of the capsule at about the same tmne 

 that the &^^ is extruded from the 

 ovarium. 



Like the spermatic capsules, the 

 ovarium (for in H. viridis there is 

 usually but one) is formed in the 

 interstitial cells of the ectoderm, but 

 nearer the fixed extremity, appearing 

 as a large round protuberance. (It 

 is shown just beginning to form at o, 

 fig. 17). With only very rare excep- 

 tions both sets of organs are formed 



upon the same individual, the sper- 

 matic capsules generally being de- 

 veloped first. When the ovarium 

 has reached a certain stage, the soft 

 protoplasm of the enclosed &^^ grad- 

 uall}^ exudes from an aperture in the 

 apex of the investing membrane (still 

 remaining attached, however, to the 

 ovarium) , and is then fertilized by 

 the spennatozoa which, about this 

 time, issue from their capsules. The 

 process of segmentation then sets in, 

 and a thick, hard shell is secreted in 

 about 4 days. After the formation 

 of the shell, the g^^^ severs its con- 

 nection with the parent Hydra and 

 sinks to the bottom. The bounda- 

 ries of the contained cell walls grad- 

 ually disappear, and by a curious 

 apparent retrogression the contents 

 of the e.^^ are once more a continu- 

 ous mass of protoplasm. In this is 

 formed a cavity which ultimately be- 

 comes the body-cavity of the young 

 Hydra. The succeeding stages of 

 development are slower, and finally, 

 usually in 6-8 weeks from the secre- 

 tion of the shell, the young animal is 

 liberated, closely resembling the 

 parent in evervthing except size. It 

 will thus be seen that Hydra has no 

 larval stage, differing in this respect 

 from all other hydroids. During 

 the period of embryonal develop- 

 ment, a great mortality is caused by 

 the growth of the mycelium of fungi, 

 by the spores of which the naked &'g^ 

 has been infected. Out of i ,500 eggs 

 gathered and studied bv Kleinenberg, 

 about 1,100 were destroyed in this 

 manner. These figures are also in- 

 teresting as an indication of the per- 

 sistent industry of this investigator. 



Hydra also multiplies by gemma- 

 tion, a process strikingly analogous 

 to that of 'budding' in plants. A 

 little swelling on its body surface 

 gradually elongates, at the free end 

 a mouth is formed, below which is 

 developed the crown of tentacles, 

 and thus a young Hydra makes its 

 appearance, the entire process being 

 usually completed in a few days. 

 (See fig. 17). The communication 



