KXUVIATION. 157 



species is about two years : whether the other and appa- 

 rently quicker-growing species, are shorter-lived, I have no 

 means of judging. 



In accordance with this rapid growth is the frequency of 

 the periods of exuviation. Mr. Thompson kept twenty 

 specimens of B. balanoides alive, and on the twelfth day 

 he found the twenty-first cast-off integument, showing that 

 all had moulted once, and one individual twice, within this 

 period.* This frequency of exuviation, together with the 

 durability of the cast-off integuments, explains the astonish- 

 ing masses of exuviae, which Mr. Peach assures me he 

 annually has observed off the coast of Cornwall ; they are 

 most abundant in April and May, but he has seen quan 

 tities also in September ; he could easily, as he tells me, 

 have filled several quart-measures with them. The speci- 

 mens sent to me consisted chiefly of Balanns balanoides, 

 perforatus, and Chthamalus stellatus. The opercular mem- 

 brane, with a narrow strip from between the two scuta, 

 and another narrow strip from between the two terga, are 

 moulted together, in connection with the more delicate 

 membrane lining the sack, and investing the plicated 

 branchiae. This membrane, likewise, is continuously 

 connected with that covering the whole body and its 

 appendages. As I have stated under the Lepadidae, the 

 inner tunic of the oesophagus, of the rectum, of the olfactory 

 pouches, and that which enters a little way into the 

 acoustic meatus, and the apodemes of the maxillae, are all 

 moulted. On the cirri and jaws, new spines are formed 

 with their upper ends enclosed within the old spines, 

 but with their lower ends projecting inwards, beyond the 

 bases of the old spines, and inverted like the fingers of 

 a glove hastily pulled off. The membranes of the body, in 

 the act of exuviation, split, I believe, only over the prosoma. 

 How the neat separation of the opercular membrane, from 

 all round the sheath and opercular valves, is effected, I do 

 not fully understand ; but it is, probably, analogous to the 

 splitting of the thick carapace of the common crab. I sus- 

 pect in Coronula, in which genus and its allies the oper- 



* In Daphnia, Straus ('Mem. du Museum,' torn, vi, p. 151) found that the 

 individuals moulted every five or six days. 



