456 



CEUSTACEA. 



slender flagellum composed of few joints. The posterior antennae 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 

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

 Eutyi)his (Typhis Risso) ovoides Eisso (Platyscelus serratus Sp. Bate), Mediter- 

 ranean,. Oxijcejjkalus piscator Edw., Indian Ocean. 



2. Sub-order : Isopoda.* 



Arthrostraca with usually broad, mare 

 or less arched body, with seven free tho- 

 racic rings, with lamellar legs function- 

 ing as branchice 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 j it terminates with a large 

 ciudal lamella. The abdominal legs are 

 only exceptionally (Tanaidce) swimming 

 feet ; as a rule they have the form of 

 branchial lamellse. 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 they are 

 hidden beneath the cephalic carapace. 

 In exceptional cases only (Apseudes) 



* H. Rathke, " Untersuchungen liber die Bildung und Entwickclung der 

 AVasserassel," Leipzig, 1832. Lereboullet, <; Sur les Crustaces de la famille 

 ces Cloportides. etc." Mem. du Museum d' hist. nat. de Strasbourg, Tom. IV., 

 IS-'O. N. Wagner. " Recherches sur le systeme circulatoirc et les organ es de la 

 respiration chez le Porcellion elanri," Ann. des sc. nat., Ser. 5, Tom. IV., 1865. 

 A. Dohrn, " Die Embryonalentwickelune: des Asellus aquations," Zeitschr fur 

 n-iss. Zool.,Tom. XVIL, 1867. N. Bobretzky, ' Zur Embryologie des Oniscus 

 murarius," Zeitxclir. fur wiss. Zool., Tom. XXIV., 1874. 



FIG. 858. Asellus aquaticu (after 

 G. O. Sars). Female with brood 

 pouch, seen from the ventral side- 



