K I PRODUCTIVE ORGANR , ;, 



function, two kinds of accessory organs, solid particlot tut- 



j>ende-d in a fluid and line hair-like filamrnt>, ar. 

 inclose relation with the n-r\,--ndiiigs. In th. < 

 both are combined, and an involution <,t ti,,- sensory region 

 takes place, which usually remains open throughout 

 represents the most rudimentary form of auditory labyrinth. 

 The Crustacean ear is the parallel of tin- In 



the Vertebrata the membranous labyrinth is .similarly an in- 

 volution of the integument, which remains open through-Mil 

 life in many fishes, but becomes shut off and Mim.unded by 

 thick mesoblastic structures in all the higher |, 

 The tympanum and the O8si<-nl<i //W/v,;.< jm - a. Mi; 

 accessory structures, formed at the expense of the hyoman- 

 dibular cleft and its boundary-walls. 



The Reproductive System. The relation of the r produc- 

 tive elements to the primitive layers of the germ is > 

 uncertain. E. van Beneden has brought forward vet 

 evidence to the effect that in ffydractinia the sp.matotoa 

 are modified cells of the ectoderm, and the ova of those oi 

 endoderm; but, whether it can be safely concluded that thi.s 

 rule holds good for animals generally, is a question that 

 only be settled by much and difficult investigation. The fact 

 that, in the Vertebrata^ the ova and spermatozoa are products 

 of the epithelial lining of the peritoneal cavity, and therefore 

 proceed from the mesoblast, appears at first sight directly to 

 negative any such generalization. But it must be ren 

 bered that the origin of the mesoblast itself is yet in 

 and that it is quite possible that one portion of that layer may 

 originate in the ectoderm and another in the endoderm. 



There is some reason to suspect that hennaphrodism was 

 the primitive condition of the sexual apparatus, and that imi- 

 sexuality is the result of the abortion of the organs of the <>' 

 sex, in males and females respectively. 



Very low down in the animal s< i i -, among the T>- 

 rid, the accessory organs of generation acquire a great com- 

 plexity. In the lower Turbell'ri<i the ebtoretoq (fad is a 

 mere short, wide passage. But, in tin- higher 7\trMhHa*od 

 Trnnatoda, the female apparatus present! _ ._ in 



which the ova are developed; vitellarian glands, whiofa give 

 rise to a supplemental or food yelk ; an oviduct : and 



vagina; and a spermathccn, in which the semen i up. 



The male apparatus presents a fcestis, I \ is, and a 



penis. The function of the vitellarian gland may be Uken on 



