C6 THE ANATOMY OF INVERTEBRATED ANIMALS. 



by cells of the ovary, or oviduct ; or accessory yelk-substance 

 may be formed within the primitive ovum itself, in the Arthro- 

 poda and in most Mollusca ; but the reproductive organs in 

 all these animals are reducible to the Turbellarian type. 



In the Annelids ( Oligochmta and Polychaeta), the ovaria 

 and testes often have no special ducts, and their products 

 make their way out of the body by canals which appear to be 

 modified segmental organs. 



In the Cephalopoda, again, the ovaria and testes part with 

 their contents by dehiscence into chambers connected with the 

 water-cavities, which are prolongations of the organs of Boja- 

 nus. And they are conveyed away from these chambers by 

 ducts, the oviducts or vasa deferentia, which commence by 

 open mouths in them. 



In the Vertebrata, the reproductive organs either dehisce 

 and pour their contents into the peritoneal cavity, w r hence 

 they are conveyed outward by abdominal pores (Marsipo- 

 branchii, many Teleostei), or they are continued into ducts 

 which open behind the anus separately from the renal open- 

 ing in the females, but in common with it in the males (most 

 Teleosteans) ; or their ducts are derived from portions of the 

 primitive renal apparatus which, as we have seen, is a struct- 

 ure of the same order as the organs of Bojanus and the seg- 

 mental organs. The testis is usually converted into a mass 

 of tubuli, which eventually open directly into the ducts [epir 

 didymis, vas deferens) derived from the renal organs. The 

 ovary, on the other hand, becomes an aggregation of sacs — 

 the Graafian follicles — and the oviducts open into the perito- 

 neal cavity. 



Development. — The embryo either passes through all 

 stages from the morula to a condition differing from the adult 

 only in size, proportions, and sexual characters, or it leaves the 

 egg in a condition more or less remote from the adult state, 

 and sometimes exceedingly different from it. In the latter 

 case, the animal is said to undergo a metamorphosis. Each of 

 these modes of development occurs in members of the same 

 group, and often in closely allied forms : as, for example, the 

 former in the crayfish (Astacus), and the latter in the lobster 

 (Homarus). 



When metamorphosis occurs, the larva may live under 

 conditions totall}' different from those under which the adult 

 passes its existence, and its structure may be variously modi- 

 fied in relation to these conditions. Thus the larva of an 



