86 CRUSTACEA. 



pairs of antennae, as well as several pairs of palpi attached to 

 the jaws and foot jaws, seems to favour the opinion that this 

 sense is transmitted hy means of these organs. The SCUM- of 

 hearing has not been distinctly proved to be possessed by 

 these animals ; it has, however, been supposed that the seat 

 of this sense was placed in an excavation observed at the 

 base of the external pair of antennae in certain of the larger 

 species, as the lobster, &c., in which cavity it has been stated 

 that a small sac filled with fluid exists, in the midst of \\liicli 

 a nervous thread was fixed : it is certain, however, that the 

 aperture of this cavity is closed by a membrane, and that in 

 some cases there exists (as in Main) a small crustaceous 

 organ or moveable opercnlnm, the use of which appears to 

 be, to distend the membrane which shuts the orifice. Of the 

 distinct existence of the sense of smell we are also equally 

 ignorant, although from analogy we are led to believe that it 

 is possessed by these animals. Its seat is also unknown, 

 although Rosenthals and Robineau considered that it existed 

 in a pair of small orifices at the base of the internal pair of 

 antennae. 



The minute Crustacea, whose growth is very rapid, change 

 their coats at very short intervals of time. Thus, Jurine 

 observed that the younger Daphniie moulted eight times in 

 seventeen days, but amongst the large species, such as the 

 crabs and lobsters, this only occurs once a year. In general, 

 the Crustacea having previously obtained such an increased 

 supply of food, as to make their shells too confined for the 

 increased size of their internal organs, they cast their entire 

 skeleton, as we may term it. Previous to doing this, they 

 appear sick, languid, and restless, seeking some secluded 

 place, in which they may securely remain until their new 

 covering shall have attained a sufficient consistence. In 

 casting their shells it is difficult, at first sight, to conceive 



