78 MESSES. NOEMAN AND STEBBING ON THE 



carcinologists, Prof. G. O. Sars, in his recent Catalogue^ of the higher Crustacea of 

 Norway. The arrangement appears to us more natural in some particulars than those 

 which have preceded it. 



Subclass ISOPODA. 



Tribe I. CHELIFEBA, G. O. Sars. 



Animal narrow, subdepressed, or subcylindrical, but never compressed. 



Head united with the first or, more rarely, with the first and second segments of the 

 peraeon, so as to form a short carapace, the surface of which is often areolated with 

 lines of depression. The fallowing segments of the peraeon are always distinct, and are 

 often separated by deep constrictions. 



The pleon is composed of six segments, the fi^rst five of which are usually distinct 

 (but in some cases coalesced into a single segment), and furnished with pleopods, which 

 consist of a basal joint and two unjointed branches (but in those cases where the 

 segments are coalesced, and sonietimes otherwise, the pleopods are undeveloped). The 

 last segment is formed by the union of the two last segments of the pleon, and is much 

 larger than the preceding, and carries the uropods. 



The eyes, when present, are situated on triangular lobes, at the anterior angles of the 

 carapace. 



The upper antenna3 are furnished with either one or two flagella. 



The lower antenna; are smaller than the upper, and are placed below them. In the 

 genus Apseudes they have an articulated antennal scale at the end of the second joint. 



The mandibles vary greatly in structure, and sometimes have, and at others have not, 

 a palp. 



The first maxillse have a backward-directed palp, which assists in causing currents of 

 water to pass through the branchial chamber, which is situated under the hinder part 

 of the carapace. 



The second maxillse are developed and setose, or rudimentary and naked. 



The maxillipeds are large, with a four-jointed palp, and also a large, membranous, 

 backward-directed branchial palp, which passes into the branchial chamber. 



The first gnathopods are largely developed, smaller in the female, but sometimes in 

 the male of great size, chelate. 



The second gnathopods in the Apseudidae are largely developed, with flattened wrist 

 and hand, margined with strong spines, so as to form a most efficient burrowing- 

 instrument ; but in the Tanaidse they are narrow and slender, and adapted for 

 progression. 



The peraeopods are formed for walking ; the two first pair are directed backwards, 



' G. 0. t^ars, ' Oversigt at' Norges Crustaceer med forclubigo Eemserkninger over de nye eller mindrc 

 bekjendte Arter,' 1882. 



