1 30 NEMATHELMINTHES 



the characteristic arrangement of contractile and medullary parts 

 described above. 



The Body-Cavity. The skin of a Nematode, as described 

 above, -contains most of the important organs of the body within 

 its thickness. The chief muscular system, the nervous system 

 with its sense organs, and the excretory organs are all embedded 

 in or form part of the skin, which in its turn encloses a cavity 

 the body-cavity in which the other two systems of organs 

 which are found in Nematodes lie. These are the digestive 

 system and the reproductive system. 



The body-cavity is continuous from one end of the animal to 

 the other, and is in no case divided up into compartments by 

 the presence of septa or mesenteries. It contains a coagulable 

 fluid with numerous corpuscles ; this is, as a rule, colourless, but 

 in Syngamus trachealis Sieb. (Fig. 70), which lives on blood, 

 the haemoglobin of its host tinges it red, though the colour is 

 said to disappear if the parasite be isolated and starved. 



The morphological nature of this body -cavity affords an 

 interesting problem. It is not a true coelom, such as exists in 

 the earthworm, since it is not surrounded by mesoderm, nor do the 

 excretory organs, with the possible exception of one or two genera, 

 open into it, nor do the generative cells arise from its walls. 

 Essentially it is a space between the mesodermic muscle-cells 

 which line the skin and the endodermic cells of the alimentary 

 canal, and although in many of its functions it resembles the 

 coelom of other animals, its morphological character is quite 

 different. 



There are no respiratory or circulatory organs in the Nema- 

 toda ; possibly the fluid in the body-cavity acts, to some extent, 

 as a carrier of oxygen, but from the inert and almost vegetative 

 life of these animals it seems probable that their respiratory 

 processes are slow, and in fact Bunge 1 has shown that Ascaris 

 mystax, found in the intestine of the cat, will live for four or five 

 days in media quite free from oxygen, and that A. acus from the 

 pike will live and exhibit movements in the same media for from 

 four to six days. 



The Digestive System. The mouth of the Nematoda is 

 usually anterior and terminal, and is surrounded by from two to 

 six projecting lips, the most common number being three. These 



1 Zeit. Physiol. C'hcm. vol. xiv. 1890, \t. 318. 



