8 THE VOYAGE OF H.M.S. CHALLENGE!?. 



latifi'ons is conspicuous by its strongly arched dorsal surface, which, together with the 

 shortness of the e|)imera, causes this species to resemble in general aspect the more 

 t^i^ical forms of the Cymothoada?. 



The outline of the l;»ody varies from oval {Serolis tuberculosa) or even pear-shaped 

 (Serolis convexa) to circular, and in the males of Serolis schythei and Serolis cormita the 

 diameter of the body from side to side is somewhat greater than the length. 



In the majority of the Isopoda the " head " segments become fused with the first 

 seo-ment of the thorax, and form a cephalic shield which is freely movable upon the 

 second thoracic segment. In Serolis the disposition of the anterior segments of the body 

 differs much from other Isojioda. The first two thoracic segments are closely united and 

 completely fused dorsally, though the sterna of the two remain distinct ; in some species 

 [Serolis schythei, Serolis necera, &c.) an incomplete transverse suture upon the first epimera 

 seems to mark the line of division between the two segments dorsally ; in others again 

 (Serolis antaretica and all the Australian species) the epimera of the two thoracic 

 segments are completely united, and show no traces of their original distinctness ; these 

 epimera are always largely developed, and completely enclose the cephalic shield on both 

 sides. The cephalic shield is very large and has the form of an heraldic shield ; it is pro- 

 longed in front into a short rostrum ; its shape varies considerably in the diff'erent 

 species, in some [Serolis necera, Serolis paradoxa) the antero-lateral portions are consider- 

 ably expanded, and the transverse diameter is greatest here and decreases posteriorly ; 

 in other species [Seivlis convexa, Serolis schythei) the cephalic shield is widest at the 

 level of the ej^es. In the majority of species the cephalic shield is sejiarated from the 

 two thoracic segments by a continuous suture ; in Serolis longicaudata this suture is 

 obsolete behind and indicates the commencement of the formation of a cephalothorax 

 composed of the head segments and the two first thoracic segments as in Tanais, with 

 which genus Serolis is considered by some to be closely connected. The five following 

 thoracic segments are free ; the eighth or terminal segment of the thorax differs from 

 that of other Isopoda in being only represented ventrally by a short sternum, wliich 

 is more or less intimately fused with that of the preceding segment, and is not prolonged 

 into epimera ; the tergum of this segment is entirely undeveloped, and the rudimentary 

 condition of the whole segment (including the appendages, which are much smaller thnn 

 those of the other thoracic segments) is interesting, inasmuch as in the Isopoda generally 

 the terminal segment of the thorax is only developed very late. 



The epimera of the thoracic segments are almost always largely developed in com- 

 parison with ntlier Isopoda; and in some of the deep-sea species [Serolis hromleyana, 

 Serolis necera) are extraordinarily elongated, and terminate in sharp spiniform points. As 

 a general rule the epimera are flat and sickle-shapt'd, curving back and gradually 

 increasing in length up to the sixth pair, which are invariably the longest ; sometimes 

 [Serolis latifrons, Serolis longicaudata, &c.) the epimera are very short, and hardly exceed 



