M4 



CRUSTACEA. 



13. Anisopoda. 



Among the Arthrostraca, the Anisopoda (Apsendes, Tanais) show 

 the most primitive condition, somewhat suggestive of that found in 

 the Schizopoda. Embryonic development and the greater part of the 

 metamorphosis here takes place within the brood-cavity of the mother 



(as in Mysis and the Cumacea). 

 The young on emerging from 

 the brood-cavity (Fig. 145), like 

 those of the Isopoda, are dis- 

 tinguished from the adult by 

 the absence of the last pair 

 of thoracic limbs. As in the 

 Cumacea, all the pleopoda, ex- 

 cept the sixth pair, which are 

 filamentous caudal appendages, 

 are wanting. A point of great 

 interest is the presence of a 

 wing-like shell-fold (ps) stand- 

 ing out laterally from the 

 cephalo-thorax. This renders 

 possible the derivation of the 

 Arthrostraca from a racial form 

 provided with a dorsal shield, 

 and also seems to establish the 

 significance of the lobe -like 

 appendages of the Asellus em- 

 bryo (cf. above, p. 151). 



l'ui. 145. — Young of Apsmdes Latreillii from 

 the brood-cavity (after Claus). 77- VII, 

 second to seventh thoracic segment ; 1-6, the 

 six abdominal segments ; a', first antenna ; 

 "", swond antenna ; ab®, sixth pleopod ; ep, 

 epipodial appendage of the maxillipede ; ex, 

 (ixopodites of the first and second thoracic 

 limbs; mxf, maxillipede; p 1 , 2' 2 , first and 

 second thoracic limbs ; ps, carapace 



14. 



Isopoda. 



We have already had occasion 

 to mention some features of the 

 development of the Isopoda 

 (p. 150), especially the dorsal 

 curvature and the development 

 of the dorsal organ in these animals. In the Isopoda also, after the 

 full number of segments have appeared and the limb-rudiments have 

 developed, the egg-envelope splits, and the larva, still incapable of 

 motion, maggot-like, and surrounded by the Nauplius cuticle, under- 

 its further development in the brood-cavity of the mother 

 (Asellus). When it leaves that cavity, it already resembles the adult 



