72 HISTOEY OF CRUSTACEA. Chap. VIII. 



is wanting in th^ young of the Wood-lice (PorcelUonides, 

 M.-Edw.) and Fish-lice (Cymothoadiens, M.-Edw.) has 

 already been noticed by Milne-Edwards. This applies 

 also to the Box-Slaters (Idothea), to the viviparous Globe- 

 Slaters (S;ph^roma) and Shield -Slaters (Cassidina), to 

 the Bopyridse (Bopyrus, Entoniseus, Cryptoniscus, n. g.), 

 and to the Cheliferous Slaters (Tanais), and therefore 

 probably to the great majority of the Isopoda. All the 

 other limbs are usually well developed in the young 

 Isopoda. In Tanais alone, all the abdominal feet are 

 wanting (but not those of the tail) ; they are developed 

 simultaneously with the last pair of feet of the thorax. 



The last pair of feet on the middle-body of the larva, 

 consequently the penultimate pair in the adult animal, 

 is almost always similar in structure to the preceding 



pair. A remarkable excep- 

 tion is, however, presented 

 in this respect by Cryjptonis- 

 cus and Entoniseus, — re- 

 markable as a confirmation 



Fig. 40. 6 



of Darwin's proposition that 

 " parts developed in an unusual manner are very vari- 

 able," for in the peculiarly-formed pair of feet there 

 exists the greatest possible difference between the three 

 species hitherto observed. In Cry;ptoniscus (fig. 39) 

 this last foot is thin and rod-like ; in Entoniseus Can- 

 in the median line of the back heUnd the seventh segment, (" behind the 

 boundary line of the first thoracic segment," Leydig), 



^ Fig. 39. Embryo of Cryptoniscus planarioides, magn. 90 diam. 



^ Fig. 40. Last foot of the middle-body of the larva of Entoniseus 

 Porcellanse, magn. 180 diam. 



