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 Orchestia and 
Talitrus, the two basal joints of the antennz 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). 


Fic. 2. 
There is no secondary appendage to the inferior 
antennz, and, with the exception of the squamiform 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 antennee. 
The flagellum in all Crustacea originates, in the upper 
antennee, after the third perfect joint; in the lower, after 
