172 INVEBTEBBATE MOBPHOLOQT. 



CHAPTER VIII. 



TYPE NEMATHELMINTHES. 



The Nemathelminths are, like the members of tlie preced- 

 ing type, characterized by the form of the body, which is 

 cylindrical and usually elongated or even thread-like, whence 

 the popular terms Round-worms or Thread-worms which 

 are frequently applied to them. The ectoderm is covered by 

 a thick layer of cuticle which it secretes, and in connection 

 with which spines, bristles, or hooks may be developed at 

 various parts of the body. There is no trace of segmentation 

 or reduplication of organs, with the exception that in some 

 forms the circular nerve-commissures uniting the longitudinal 

 cords may succeed each other with tolerable regularity ; the 

 cuticle, it is true, especially when thick, is ringed by numer- 

 ous grooves succeeding one another at short intervals, but 

 this cannot be interpreted as an indication of metamerism, 

 but is more probably a provision to counteract the rigidity of 

 the cuticle and to give a considerable amount of mobility to 

 the body. The Nemathelminths accordingly have the same 

 grade of individuality as a simple Platyhelminth, such as an 

 Alloiocoelan, and are to be regarded as metamere individuals. 



One important difference of structure which these worms 

 show from the Platyhelminths is the presence of a capacious 

 ccelom, the interval between the digestive tract and the mus- 

 culature of the body-wall not being filled up by parenchyma- 

 tous mesoderm, but being a simple undivided cavity in which 

 lie the reproductive organs. These latter are simple, the 

 animals being as a rule bisexual, and there is no separation 

 of the female organ into ovary and vitellarium. Structures 

 of an excretory nature occur in one of the two classes into 

 which the type is divisible, but a blood vascular system is 

 entirely wanting. 



The habit of life varies greatly in the various members of 



