Characteristics of Annulata. cxxvii 



no alternation of generations is observable, the history of the evolu- 

 tion of the embryo is much complicated by the fact that two ' hosts' 

 are necessarily required for its harbouring" and sustentation at dif- 

 ferent periods ; in one only of which, usually a vertebrate animal, 

 the sexnal condition can be attained to. The enormous quantities 

 of ova which parasitic A^ermes produce, stand in direct relation to 

 the very great difficulties which their peculiar mode of life opposes 

 to the continuation of the si3ecies. 



Vermes of the same species in the higher orders of the sub- 

 king'dom appear to be competent to the maturation of sexual 

 products at very different ages. In thus assuming sexual functions 

 before attaining their full size, these Vermes resemble the Fishes 

 in the Vertebrate Sub-kingdom, but differ not only from the air- 

 breathing classes of Arthropoda, but also from the lower members of 

 their owti Sub-kingdom. 



It is sometimes said that the power of repairing injuries and 

 mutilations which distinguishes this sub-kingdom as a whole in 

 an eminent degree, is connected with the possession of the faculty 

 of metagenesis. The power of repair however is very great in the 

 terrestrial Oligochaeta s. Lumhricidae, in which metagenesis has 

 not been observed ; and though the two faculties are both alike 

 absent in the Nematoidea and in the Discopliora^ it is better to 

 explain this fact by a reference to the special habits of these animals, 

 which, as testified to by the universal presence in the one, and the 

 ver}" common presence in the other of caudal suckers, would appear 

 to be more or less incompatible with reproduction by gemmation. 



Class, Annulata proper. 



Vermes of elongated and almost always cylindrical shapes, made 

 up of a series of segments, which are homonomous except at either 

 extremity of the body. They are annulated externally, and inter- 

 nally their perivisceral cavity is divided by dissepiments into more 

 or less perfectly separated compartments and chambers. They all 

 possess a chain of ventral in addition to, and in commissural junc- 

 tion by a circum-oesophageal collar with a prae-oral cerebroid 

 mass ; the digestive tract is in all of them proctuchous, and, with 

 the exception of the lUrudineae, suspended in a large perivisceral 

 space. With a few exceptions they possess a closed system of 



