XX 



INTRODUCTION. 



Nearly connected with the Sand-hopper and its allies (consti- 

 tuting the group Amphipoda) is the group Isopoda, represented 

 by the Wood-lice and the related marine forms. These have 

 the segments of the body and their appendages constructed 

 on the same general plan as the Amphipoda; but the body is 

 usually compressed from above downwards instead of from side 

 to side, the first three pairs of thoracic appendages are usually 

 turned forwards and the last four backwards, the pleopoda are 

 delicate lamellae which serve as gills, and more or fewer of the 

 segments of the abdomen are usually coalescent. 



Another group of the Malacostracous Crustacea, differing 

 considerably from the preceding, is that formed by the Squillm 

 and their allies the Gonodactyli. The body of Gonodactylus 

 chiragra (Fig. vii.)— the commonest Australian representative of 

 this group— is elongated like that of the Prawn, but, instead of 

 the carapace covering all the thorax but one segment, as in the 

 latter, it is very small, and covers only the segments of the head. 

 Moreover, in Gonodactylus the first two segments, the ophthalmic 

 and antennulary somites, are movable on one another, and lie 

 on the same line with the rest of the body, whereas we have seen 

 that in the Prawn the anterior segments have their sterna bent 



FlG. VII. — Qonodaciylus chiragra, natural size. 

 th, Thorax; abd, abdomen ; i.-viii., segments of the thorax; i -vi., segments 

 of the abdomen ; o, eye ; d, upper antenna? j d 1 , lower antenna? j 1-8, thoracic 

 a, b, c, d,f, appendages of the abdomen; x, telson. 



upwards, and amalgamated with those of the rest of the cephalo- 

 thorax. The eyes in Gonodactylus (Pig. vii., o) are stalked. The 

 antennules (Pig. viii., a 1) have a three-jointed peduncle support- 

 ing three short, multi-articulate flagella ; the antennae (Pig. viii., 

 d 2) have a single, short, stout ftagellum, and bear at the base a 

 large scaphocerite or squamous appendage. The mandibles (Mm) 

 are divided at the extremity into two obscurely toothed rami. The 



