ANURIDA. 379 



cuticle of the body-wall. The dilatores ani are a similar 

 series of muscles, situated immediately around the anus. 



The anus is situated on the last segment of the body, 

 and is surrounded by three papillae, the largest of which 

 is median and dorsal and the other two ventro-lateral in 

 position (Plate I., fig. 3, an.). 



5. The Muscular System. 



In order to explain the muscular system at all 

 adequately a very lengthy description would be necessary, 

 and such an account is beyond the scope of this Memoir ; 

 only its general features, therefore, will be referred to. 



The musculature of the abdomen may be broadly 

 divided into three systems. Firstly, a series of longitu- 

 dinal tergal muscles, which are arranged in the form of an 

 outer and an inner set on either side of the dorsal vessel 

 in each segment (I. t. m. in figs. 50 and 51). Secondly, 

 and corresponding with these on the ventral side, a system 

 of longitudinal sternal muscles (I. s. m.) ; and thirdly, a 

 system of tergo-sternal muscles, some of which pass 

 vertically and others obliquely from the tergum to the 

 sternum in each segment (t. s. muse). 



In the thorax this general arrangement is adhered to, 

 but it has undergone modification in conformity with the 

 requirements of the mobility of the legs. 



In the head the musculature is very complex ; it is 

 principally concerned with the movements of the various 

 mouth-parts. In addition to the usual muscles in connec- 

 tion with those organs, there are special muscles for 

 drawing the mandibles and first maxillae forwards, and 

 partially protruding them through the mouth, and there 

 is also an antagonistic set for retracting these organs back 

 again into the cavity of the head. In Anurida more than 



