1900.] 



HOMCEOSIS IN THE GENUS ASELLUS. 



269 



a number of Aselli taken from a large aquarium in the Cambridge 

 Zoological Laboratory for examination by the students. 



As the figure shows (fig. 1), the abnormal structure stands exactly 

 as the left antennule should. Both antennae aud the right anten- 

 nule are normal, as also are the mandibles, the other mouth-parts, 

 and the rest of the body and its appendages, so far as they could 

 be observed. The animal was of good size. 



Fig. 1. 



l.ant 2 .. 



Head of Asellus aquations, seen from dorsal side. — rt. ant. 1 , right antennule ; 

 rt. ant?, I. ant. 2 , right and left antennae ; r p, palp of normal right man- 

 dible projecting from below; p', palp of the abnormal mandible; trit, 

 triturating process. 



The abnormal appendage (fig. 2, p. 270) is a fairly well-formed 

 mandible. The blade is complete, having two toothed processes aod 

 eleven plumose setae. The latter are not quite regularly placed. 

 The presence of two toothed processes proves that the appendage is 

 a true left appendage, for the right mandible in Asellus aquaticus has 

 only one toothed process, which bites between the two toothed 

 processes of the left mandible l . 



The teeth on these processes are more numerous and less regular 

 than in a normal mandible. 



1 Sars, G. O., Crustacea d'Eau douce de Norvege, p. 97, pi. yi'n. fig. 26. 



