Exuviation in the Crustacea. 349 



upper half is invested by a shelly covering, its exuviation is 

 limited in correspondence, there being nothing to separate 

 from the lower, the fleshy portion. The peculiarities of exu- 

 viation by the other crustacean genera, being sought from 

 themselves, will be found extremely interesting by the 

 practical naturalist. 



In as far as I have been able to ascertain, prolific females 

 are exempt from exuviation during the period of gestation. 

 The spawn or roe adhering externally to the shell would be 

 endangered by such a process. This spawn, which is seen 

 in beautiful variety in colour and quantity, often resembles 

 luxuriant clusters of currants or grapes, each capsule con- 

 taining a fcetus, which is discharged on maturity, while the 

 spawn still remains in situ. The young has no resemblance 

 whatever to the parent. 



The preceding facts, combined with many other observa- 

 tions, lead to the following conclusions : — 



1. The crustacean tribes are invested by a rigid inexpan- 

 sible shell. 



2. As the existing shell cannot dilate to allow the incre- 

 ment of the animal, it is wholly cast off by exuviation^ to 

 make way for another, which is always of larger dimensions. 



3. This exuviation, commencing at very early age, is 

 repeated at irregular intervals during the progress of incre- 

 ment, each successive shell with its animal exceeding the 

 size of its precursor. 



4. The larger or new shell and animal are generated or 

 regenerated within the existing shell, which opens for its 

 exit when mature. 



5. No enlargement of the new subject is sensible after 

 production. 



6. Whatever the mutilation may be which the existing shell 

 and animal have undergone, the new subject is always pro- 

 duced entire and perfect by exuviation. 



7. Prolific females are exempt from exuviation, that their 

 spawn adhering externally may not be exposed to injury. 



8. The young of many crustaceans, which bear no resem- 

 blance to the parent, attain symmetry and perfection through 

 the medium of successive metamorphoses. 



