A history of the British sessile-eyed Crustacea . Fig. 1. tubercle situated on the ventral surface of the cephalonin the Brachyura, laterally anterior to the oral apparatus,and indeed covered by some of the appendages, in thehigher groups of the class. The denticle in the Amphipoda,upon close examination, appears to have an open extremity,through which a cylindrical tube, retained in its place bymembranous ligatures, protrudes. This tube closes at the INTRODUCTION. XI internal extremit)- rather suddenly, and encloses theelongated bulbous extremity of a nerve-thread, that pro-ceeds from a secon
A history of the British sessile-eyed Crustacea . Fig. 1. tubercle situated on the ventral surface of the cephalonin the Brachyura, laterally anterior to the oral apparatus,and indeed covered by some of the appendages, in thehigher groups of the class. The denticle in the Amphipoda,upon close examination, appears to have an open extremity,through which a cylindrical tube, retained in its place bymembranous ligatures, protrudes. This tube closes at the INTRODUCTION. XI internal extremit)- rather suddenly, and encloses theelongated bulbous extremity of a nerve-thread, that pro-ceeds from a second bulb or nerve-ganglion implanted atthe base of the denticle. This denticle, though frequent,is not invariably present. In the genera Orcheslia andTalitrus, the two basal joints of the antennae are builtinto the anterior wall of the cephalon, so as to begenerally mistaken for it; while in others, as also in theIsopoda, every trace of the denticle is lost (Fig. 2).. Fig. 2. There is no secondary appendage to the inferiorantennae, and, with the exception of the squamiforra platein the Macrura, it is never found in Crustacea; nor is itinvariably a macrurous condition, since in some genera itis entirely absent; and even in Palinurus, a most typicalform, it is lost as an appendage, being distinguishableonly in the outline impressed in the walls of the fourthjoint of the antennse. The flagellum in all Crustacea originates, in the upperantennae, after the third perfect joint; in the lower, after Xll INTRODUCTION. the fifth; and in every case the secondary appendage,whether in the form of a scale attached to the lower, or afilamentary appendage, or seA^eral, invariably in upper andlower alike arises from the distal extremity of the third. This appears to be a very constant condition withall the appendages of the cephalon, pereion, and most frequent exception exists in the fiist jointor coxa, as exemplified in the branchial appendages andthe ov
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Keywords: ., bookauthor, bookcentury1800, bookdecade1860, booksubjectcrustacea
