302 REPOET OF COMMISSIONER OF FISH AND FISHERIES. 



latcd to the inferior surface of the pleon as to fold together like a pair 

 of cupboard doors, forming an operculum for the protection of the more 

 delicate pleopods. Except in the Tanaidce, respiration is carried on by 

 means of the pleopods. 



In the AsellidcL', Idoteidcc, and some other families two or more of the 

 segments of the pleon are united, so that, seen from above, the pleon, 

 like the head, may appear to consist of a single segment, as in Jeer a albi- 

 frons (pi. I, fig. 4), but the number of pairs of its appendages, usually 

 six, remains as evidence of this consolidation. In like manner the 

 head is to be regarded as composed of several segments united, and the 

 number of such segments is indicated by the number of pairs of appen- 

 dages. In the Tanaidce and many of the higher Crustacea, the eyes, 

 more or less distinctly stalked or articulated with the head, are seen to 

 be of the nature of a pair of appendages, which may be regarded as be- 

 longing to the first cephalic segment. The antennulae and antennae 

 represent, respectively, the second and third cephalic segments, and, in 

 like manner, the mandibles and two pairs of maxilla3 represent the 

 fourth, fifth, and sixth segments of the head. A seventh segment is 

 indicated by the maxilh'peds. This segment is regarded by Huxley as 

 properly thoracic* instead of cephalic, but, for purposes of description, 

 the segment and its appendages will be regarded as belonging to the 

 head, and the next segment considered the first thoracic. 



This segment, like the following thoracic segments, is usually free, 

 and has the dorsal region well developed, but in the adult GnatMa it 

 is united with the head, and still more closely so in the Tanaidce. The 

 seventh thoracic segment is the last to develop, and in young Isopoda, 

 taken from the incubatory pouch, only six pairs of legs are commonly 

 found. In GnatMa this condition prevails through life, and in the adults 

 the first pair of legs are also modified, especially in the males, so as to 

 quite lose their pediform character, leaving apparently only five pairs of 

 legs. Further modifications of structure will be described in the fam- 

 ilies and genera in which they occur. 



The nomenclature adopted, as explained above, corresponds nearly 

 with that proposed by Mr. C. Spence Bate in his Report on British 

 Edriopnthalma, and used by the authors of the British Sessile-eyed 

 Crustacea. 



The length of an Isopod, in the present article, is given as the length 

 of the body, exclusive of appendages, and is measured from the front 

 of the head to the tip of the pleon. When, as in Janira, the head is 

 produced medially into a ''rostrum" (see pi. II, figs. 9 and 10), the meas- 

 urement is taken from the tip of the rostrum, which is a part of the 

 head, and not properly an "appendage." 



Among the Edriophthalma or sessile-eyed Crustacea, the Isopoda may 

 in general be characterized as follows: Body depressed rather than 

 compressed ; respiration carried on by means of the pleopods, of which the 

 last pair only are modified into uropods. 



* Huxley, Anat. Inv., Am. ed., p. 276. 



