NO. 2194. NORTH AMERrCAN PARASITIC COPEPODS— WILSON. 25 



It will be of great interest to find the larvae of Peroderma and 

 n aemohaphoides and ascertain if they also show a reduction in the 

 number of legs, the latter genus having only two pairs. 



INTERNAL MORPHOLOGY. 



The body icall. — The body wall is composed of two laj'ers — an 

 outer transparent layer, which may or may net become chitinous, 

 and an inner opaque and cellular layer, the chitinogen membrane 

 of Claus and other authors. 



The outer layer is made up of thin lamellae, lying one upon 

 another, with no intervening spaces. In Lernaea harnirnii Hartmann 

 described quite a complicated pattern of raised sculpture on the ex- 

 ternal surface of the outer layer. This may be seen on fresh and liv- 

 ing specimens but usually disappears during preservation. Through 

 this outer layer run pore canals, which vary in proximity in different 

 parts of the body, being closest together along the center of the 

 body and farthest apart on the antennae, swimming legs, and furca. 

 These canals are connected more or less intimately with the soft 

 inner layer of the wall and probably function in excretion. 



The inner layer of the body wall is softer than the outer and is 

 never hardened into chitin. It is composed of polyhedral cells, which 

 vary greatly in thickness, those lining the posterior body and abdo- 

 men being much thicker than those found in the anterior body and 

 arms. The cells are not much flattened but are more or less inflated 

 and filled with a fine-grained brownish substance, glandular in 

 nature, and having spherical nuclei with small nucleoli. This layer 

 normally lies in contact with the inside surface of the outer layer, 

 but in alcoholic specimens the two la3'ers are often separated. In 

 the living animal the cells of the inner layer undergo certain 

 changes, thus described by Ilartmann : " Some cells break loose, 

 stretch out, become narrower, take on the appearance of threads, 

 anastomose with one another, and send out many irregular processes " 

 (p. 73G). This wandering of the cells takes place in all parts of 

 the body, in the horns, and in the appendages. As a result they form 

 a meshwork of active protoplasm over the inside surface of the inner 

 layer and fill such spaces as the inside of the abdomen around the 

 intestine. 



This meshwork is particularly thick inside the posterior body of 

 the copepodid larva and yrobably contributes greatly to the nour- 

 ishment and increase of that part of the body during its rapid 

 development. 



The muscular system. — In the copepodid stages the muscular sys- 

 tem is well developed and very closely resembles that of the Caligidae, 

 thus furnishing another evidence of the close relationship of the two 

 families. But as soon as the female has fastened herself to her final 



