140 



ZOOLOGY. 



branous vesicles fixed to the base of the limbs, under the 

 thorax ; and these perform the functions of branchias. Finally, 

 in the Crustacea isopoda, respiration is accomplished by 

 means of false abdominal limbs, which are foliaceous and 

 membranous. 



p > - 



p" p" b pa pa g 



Fig. 429. Squill (Shrimp).' 



569. There exists a very small number of these animals 

 which live in air ; but they form an exception to what we 

 have already said relative to the differences of structure of 

 the respiratory apparatus in aquatic and terrestrial animals : 

 for in place of being furnished with lungs or tracheae, they 

 breathe by branchiae, like the first ; only these organs are dis- 

 posed in such a way as to maintain themselves in a moist 

 state, required for the exercise of their functions. The ge- 

 carcins or land crabs (Fig. 430), met with in various regions of 

 the globe, but abounding especially in the Antilles, where 

 they are known by the name of totirl&urous, present a 

 remarkable example of this anomaly. In place of living in 

 water as the ordinary Crustacea, they are terrestrial, and 

 although they have gills, some of them become asphyxiated 

 rapidly by submersion. Their respiration is, in fact, too 

 active for the small quantity of oxygen dissolved in water 

 sufficing for their wants; whilst in the air, they of course find 

 this material in abundance ; and a disposition analogous to 

 that which we have already met with in fishes (Fig. 318), per- 

 mits them to remain out of the water without their branchiae 

 drying up so as to become unfit for the performance of their 



* y, eyes ; a, antennae ; p', limbs of the first pair ; p", limbs of the three 

 following pairs ; p'", thoracic limbs of the three last pairs ; pa, false abdo- 

 minal limbs; b, branchiae ; g, caudal fin. 



