MICROSCOPICAL STUDIES. 47 



By these complicated phases Sacculina becomes primarily an 

 internal parasite, bat as the size of the sac gradually enlarges, it 

 exercises so great pressure upon the integument of the abdomen of the 

 host as to cause such thinning as permits the sac to burst its way 

 through, and to appear as an external parasite. It is probable that 

 the internal stage is assumed to obviate the -danger of being thrown 

 off and thereby destroyed, on the occasions of its host's moulting 

 during the early period of the attachment and before its roots have 

 had time to ramify extensively. It is significant of the paralizing 

 effect exercised upon the growth of the crab, that once the sac has 

 become external, i.e. when it has reached adult life, with its roots 

 ramifying extrusively through the viscera, that moulting ceases, 

 the crab remaining stationary in size. 



Occasionally I have found two Sacculince parasitic upon the 

 same crab. An interesting feature in Sacculina, is that, although 

 hermaphrodite, complimental males exist, located usually around the 

 cloacal aperture of the sac. They have a Cypris-like appearance. 



The three species of Cirripedes above described are thus all 

 connected by similarity in larval history (ontogeny). The diverse 

 adult forms are also linked together by a gradation of intermediate 

 types of great interest, that throw much light on the evolution of the 

 more changed. Thus the change from the stalked Lepas to the 

 sessile Balanus, can be understood by reference to ScalpeUum, a 

 stalked genus where the peduncle is reduced and the number of the 

 calcareous plates of the mantle so augmented, some being intercalated 

 between the terga and scuta and the border of the peduncle, that if 

 the latter be lost and the carina and certain of the additional plates 

 join laterally and arrange themselves as a circular wall pushing away 

 the terga and scuta, we obtain the modification seen in Balanus — 

 a ring of plates with the opening closed, lid-like, by the four plates 

 of the terga and scuta. To the sac-like and limbless form of 

 Sacculina, the gap is largely bridged by our knowledge of such 

 genera as Alcvppe and Gryptophialus, degenerate forms enveloped in 

 bag-shaped mantles, and provided with but three pairs of cirriform feet, 

 and which live parasitically in holes bored in shells. More degenerate 

 still is Proteolepas, a remarkable grub-like form living in the mantle 

 cavity of other Cirripedes ; limbs are entirely absent and the 

 digestive tube is rudimentary. 



Classification : — 

 Class.— CRUSTACEA. Sub-Class.— Entomostraca. 

 Order. — ClRRlPEDlA. 



Sub-Order I. — Thoracica ; thorax always present, usually provided 

 with cirriform feet ; mouth and alimentary canal 

 present. 



