268 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



the anus is situated, being completely membranous, and the 

 sides only being strengthened by calcareous plates extend- 

 ing inward from the dorsal hard skeletal element, or sclero- 

 derniite. 



The powerful tail-fin of the Astacus is formed by the tel- 

 son, combined with the two distal divisions of the sixth ab- 

 dominal appendages on each side. The other abdominal 

 appendages can have very little influence on locomotion. In 

 the female, however, they play an important part as the car- 

 riers of tlie eggs ; and in this sex there is nothing worthy 

 of special notice about the first and second abdominal somites 

 or their appendages, except that those of the first are rudi- 

 mentary. In the male the appendages of these two somites 

 have undergone a very interesting metamorphosis, whereby 

 they are fitted to subserve copulation. Those of the second 

 somite (Fig. 71, I) are enlarged, and the protopodite and 

 basal joint of the endopodite are much elongated ; the latter 

 being produced internally into a plate rolled upon itself, and 

 thence concave outward and forward. It is as long as the rest 

 of the endopodite (which, like the exopodite, is many-jointed), 

 and serves as a sort of sheath for the reception of the append- 

 age of the first abdominal somite (Fig. 71, H), which con- 

 sists of a single plate rolled upon itself in a similar manner, 

 so as to resemble a grooved style. These organs, doubtless, 

 help to convey the spermatophores from the male genital 

 apertures to the body of the female. 



The compact and firm cephalo-thorax seems at first to dif- 

 fer widely from the flexible, many-jointed abdomen ; but the 

 most posterior of its somites offers an interesting transition 

 from the one to the other. This somite is, in fact, only united 

 by membrane to that which precedes it, and is hence, to a 

 certain extent, movable. Its sternal portion is completely 

 calcified, but the epimera ' are only partially calcified. 



The appendages of this somite differ widely from those of 

 the abdomen, representing (as their development shows) only 

 the protopodite and endopodite of the latter. Each is a long, 

 firm leg, composed of seven joints, the proximal one being 

 thicker than any of the rest, while the terminal joint is nar- 

 row, curved, and pointed. To these seven joints Milne-Ed- 

 wards has applied the following terms (Fig. 71, G) : The 

 proximal one, which articulates with the somite, is the coxo- 



1 The term epimeron is here employed in a more special sense than that 

 commonly used, to denote that part of the lateral wall of a somite which is 

 situated between the articulation of the appendage and the pleuron. 



