514 CRUSTACEA MALACOSTRACA. 



the posterior limit not of the head but of the regions corresponding to the 

 antennary and mandibular appendages, but the great variation in its 

 position even in allied genera, its incompleteness in some forms, and its 

 entire absence from many of the lower Decapods, render its segmental 

 nature very doubtful. 



Other grooves are often present on the decapod carapace, but as in the 

 case of the cervical groove their morphological significance is obscure. 

 These are well marked and may present a complex arrangement * in the 

 Thalassinidea. Among them may be mentioned a pair of open longi- 

 tudinal sutures dividing the median from the lateral regions of the cephalo- 

 thorax, and known as the lineae thalassinicae. They are present in 3 out 

 of the 4 families. 



The projecting margin, or pleural fold of the carapace, is found 

 in varying relations with adjacent parts in the several groups of 

 the Decapoda. These must be considered in some detail. 



Posteriorly the pleural fold forms the outer or lower boundary 

 of the gill chamber and is known as the branchiostegite. An- 

 teriorly it abuts against the sides of the oral apparatus, and 

 in the Brachyura and Anomura, where the mouth regions have 

 undergone specialization, the anterior part of the pleural fold of 

 either side is separated by a pleural suture from the rest of the 

 carapace and is known as the pterygostomial region (Figs. 315 and 

 316, Pt.s). 



In the lower Macrura the sides of the carapace are little differen- 

 tiated. The pleural fold extends forward to the region of the 

 eyes, where it is excavated to allow of the play of the eye stalks, 

 and anteriorly it is continued into the base of the rostrum. 



The modifications to which it is subject in the higher 

 Decapods are associated with modifications of the sternal region 

 of the body in front of the mouth, known as the epistome (cf . Figs. 

 315 and 316). This area is bounded behind by the insertions 

 of the mandibles and the base of the upper lip or labrum ; the 

 side limits are set by the pleural margins of the carapace and 

 the anterior by the bases of the first and second antennae. In 

 front, between the bases of the first antennae, the epistome 

 often ends in a more or less prominent point in the middle line 

 (Fig. 316). In the Caridea the epistome is of small antero- 

 posterior extent, but in the Macrura Reptantia the length in 

 this direction is considerably greater. In the Nephropsidae its 

 pleural boundaries though clearly marked are low. In the 



* Cf. Borradaile, Marine Crustaceans, Pt. xi. Gardiner's Fauna and 

 Geography of the Maldive and Laccadive Archipelagoes, vol. ii, p. 690. 



