INTRODUCTION. 



XI 



internal extremity rather suddenly, and encloses the 

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

 ceeds from a second bulb or nerve-ganglion implanted at 

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

 is not invariably present. In the genera Orchestra and 

 Talitrus, 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). 



V' 





FIG. 2. 



There is no secondary appendage to the inferior 

 antenna?, and, with the exception of the squarniform plate 

 in the Macrura, it is never found in Crustacea ; nor is it 

 invariably a macrurous condition, since in some genera it 

 is entirely absent ; and even in Palinurus, 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 antennae. 



The flagellum in all Crustacea originates, in the upper 

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



