NOTES ON PARASITES. 

 II. 



THE parasitic habit of life is usually associated with 

 certain structural peculiarities which present them- 

 selves in a greater or less degree in all the Entozoa. 

 Among such peculiarities the most striking are (i) the 

 presence of hooks and suckers or other objects of ad- 

 hesion ; (2) the shape of the body, adapted for insinuating 

 itself into various spaces of the unfortunate host or for 

 boring into his tissues, and in those parasites which live in 

 such passages as the alimentary canal, being so moulded 

 as to offer little resistance to the passage of food so that 

 the intruders are not readily dislodged ; (3) the tendency 

 of the sense organs and nervous system to diminish ; (4) 

 the corresponding increase in complexity both in the 

 structure of the reproductive organs and in the details of 

 the development of their products ; and finally (5) the 

 tendency to do without a functional alimentary canal. 



There are perhaps two other structures in the bodies of 

 the larger Entozoa which show profound modifications pos- 

 sibly connected with their endo-parasitic mode of life. 

 These are the epidermis and the body-cavity. In a former 

 number of this journal, 1 I have described the condition of 

 the skin in Cestodes and Trematodes as interpreted by Prof. 

 Blockmann of Rostock. The condition of the epidermis 

 in Nematodes and in the Acanthocephala is even more re- 

 markable. Within the clearly defined cuticle which covers 

 the bodies of these animals, is a layer of protoplasm more 

 or less differentiated into minute fibrils and scattered 

 through the substance of this layer are a certain number 

 of nuclei. No cell limits are to be seen although they 

 exist in the early stages of the development of the embryo. 



This peculiar tissue is in the Nematodes heaped up 

 into four ridges which run down the body, one dorsal, one 



1 Science Progress, new series, vol. i., p. 78. 



