CRUSTACEANS 



405 



mouth, the two pairs of feelers are directed forwards, an ad- 

 vantageous position for sense organs of the kind. The second 

 or smaller pair, the antennules, conform markedly to the Y- 

 diagram, and consist of a basal stalk and slender outer and 

 inner branches. But it is by no means certain that we are 

 justified in comparing the parts of this appendage with those 



FEELER 2 



FOOT JAW 3 -fS 



FIN 



Fig. 248. Appendages of Lobster (Homarus vulgaris} 



PR. Base of appendage (protopodite); EN. inner branch (endopodite) ; EX. outer branch (exopodite), (the right- 

 hand EX. in feeler 2 indicates excretory aperture; EP. gill-plate (epipodite); G, gill. 



of the other ones, for it must be noted that the basal stem is 

 here three-jointed instead of two-jointed as in all the other cases. 

 The large feelers or antennce conform to the type, for they con- 

 sist of a two-jointed stalk, to which are attached a scale-shaped 

 outer branch, and an excedingly long inner branch, which can be 

 swept round so as to explore a considerable area in the neighbour- 

 hood of the body. 



We have in the Lobster appendages an excellent example 

 of the principles enumerated on p. 195, whereby structures of 

 generalized type are modified in various ways to bring about 

 special ends. These principles are: i. variations in shape'. 



