Fig. 4. A. oomrmmis: A, dactylus and palm of first peraeopod; B, 

 first peraeopod. From Williams (1970). 



coxa, is not distinct from the thorax. Dorsally, the thorax is covered 

 by a tergal plate, which in the second to seventh segments is expanded 

 to a small lobe at each anterolateral angle. Breeding females bear 

 ventral ly a series of flaps, oostegites, which arise from near the in- 

 side of the base of the anterior legs and form a brood-pouch or marsu- 

 pium. In non-breeding females, their place is taken by small, club- 

 shaped structures. In males, a pair of tube-like penes arise from the 

 posterior, ventral margin of the last thoracic segment; the penes pro- 

 ject backwards. 



The abdomen is conspicuously dorsoventrally flattened. In males, five 

 pairs of pleopods are borne ventrally, of which the anterior two pairs 

 are small and serve a sexual function (Fig. 6A, B) . The remaining pleo- 

 pod pairs (3-5, Fig. 7A-C) are large, plate-like, and serve a respira- 

 tory function; each of these pleopods consists of a larger, more robust 

 and variously setose lower plate (exopod) , and a smaller, fragile, non- 

 setose upper (or inner) plate (endopod) . In females, only four pairs 

 of pleopods occur (the true first pair are always absent) of which the 

 most anterior pair consists of small and simple triangular plates, 

 whilst the remaining three pairs are similar to those in males. 



Of greatest importance in the identification of Asellus species is the 

 structure of the first and second male pleopods, and of critical impor- 

 tance is the conformation and shape of the tip of the endopod of the 

 second pleopod; especially marked variation between species is dis- 

 played in the structure of the endopod tip. It is for this reason that 

 these appendages are discussed further below firstly for A. aommunis in 

 particular, and secondly, with respect to the conformation of the tip of 

 the endopod of the second pleopod alone, for all North American species 

 of Asellus in general. 



