456 



CBUSTACEA. 



slender flagellum composed of few joints. The posterior antennas are in the 

 male very long and folded three to four times together in a zigzag fashion ; in 

 the female they are short and straight, sometimes quite reduced. The basal 

 joints of the fifth and sixth pairs of legs are usually enlarged into great 

 lamellae, which cover the thorax. The seventh pair is generally rudimentary. 

 Entij2)hix (Tt/j)Jris Risso) ovoidex Risso (Platysci'lus -vet-ratios Sp. Bate), Mediter- 

 ranean. OseycepJutlus pisoator Edw., Indian Ocean. 



2. Sub-order: Isopoda/ :: 



Arthrostraca with usually broad, more 

 or less arched body, ivith seven free tho- 

 racic rings, with lamellar leys function- 

 ing as -brancldcn on the short-ringed, 

 often reduced abdomen. 



The structure of the body, which is 

 flat in shape and covered by a hard, 

 usually encrusted integument, presents 

 a great agreement with that of the 

 Amphipoda, to which the in many 

 respects peculiar Tanaidce are most 

 nearly allied. The abdomen of the 

 Isopods is, however, usually much short- 

 ened and composed of six short seg- 

 ments, which are often fused with one 

 another ; it terminates with a large 

 caudal lamella. The abdominal legs are 

 only exceptionally (Tctnaidce) swimming- 

 feet ; as a rule they have the form of 

 branchial lamellae. The sixth pair may 

 be fin-like or styliform. The anterior 

 antennae are, with a few exceptions, 

 shorter than the posterior and external 

 antennae ; in rare cases (Oniscidce) they 

 become so much reduced that the}' are 

 FIG. 358. AseUnt aquaticus (after hidden beneath the cephalic carapace. 



G. O. Sars). Female with brood . , 



pouch, seen from the ventral side. In exceptional cases only (Apseudes) 



* H. Rathke, '' Untersuchungen Ubcr die Bildung und Entwickelung dtjr 

 Wasserassel," Leipzig, 1832. Lereboullet. ''Sur les Crustaces de la famille 

 des Cloportides, etc," Mem. du Mvxeuni d'liht. >txt. dc Strtixln>iu-(/, Tom. IV.. 

 1850. N. Wagner. " Eecherches sur le systeme circulatoire et les organ us du la 

 respiration chez le Porcellion elargi," Ann. di'x xc. nat., Ser. 5, Tom. l\.. ]><>.";. 

 A. Uohrn, " Die Embryonalentwickelung des Asellus aquations," Zeitxehr filr 

 wl*x. Z<>ol.,Tom. XY1T., 1867. N. Bobretzky, " Zur Embryologie des Oniscus 

 murarius," Zritxclii-./iir iris*. Zool., Tom. XXIV., 1874. 



