INTRODUCTION. xi 



internal extremity rather suddenly, and encloses the 

 elongated bulbous extremity of a nerve-thread, that pro- 

 ceeds from a second bvilb or nerve-ganglion implanted at 

 the base of the denticle. This denticle, though frequent, 

 is not invariably present. In the genera Orchestia and 

 Talitrns, the two basal joints of the antennae are built 

 into the anterior wall of the cephalon, so as to be 

 generally mistaken for it; while in others, as also in the 

 Isopoda, every trace of the denticle is lost (Fig. 2). 



There is no secondary appendage to tbe inferior 

 antennse, and, with the exception of the squaraiforra plate 

 in the Macrui'a, it is never found in Crustacea ; nor is it 

 invariably a macrurous condition, since in some genera it 

 is entirely absent ; and even in Palinuriis, a most typical 

 form, it is lost as an appendage, being distinguishable 

 only in the outline impressed in the walls of the fourth 

 joint of the antennse. 



The flagellum in all Crustacea originates, in the upper 

 antennae, after the third perfect joint ; in the lower, after 



