DECAPOD CRUSTACEA. 



549 



ceived from others, show that there is no essential difference be- 

 tween the early forms of the sessile (Balanidae, or " acorn-shells") 

 and of the pedunculated Cirrhipeds (Lepadidse or "barnacles"); 

 for that both are active little animals (Fig. 277, A), possessing 

 three pairs of legs and a pair of compound eyes, and having the 

 body covered with an expanded shield, like that of many Ento- 

 mostracous Crustaceans, so as in no essential particular to differ 

 from the larva of Cyclops (Fig. 276, c). After going through a 

 series of metamorphoses, one stage of which is represented in 

 Fig. 277, B, c, these larvae come to present a form D, which re- 

 minds us strongly of that of Daphnia ; the body being enclosed 



FIG. 277. 



Development of Balanus balanoides:*., earliest form: B, larva after second moult ;- 

 of the same ; D, stage immediately preceding the loss of activity; a, stomach (?) ; 

 future attachment (?). 



3, side view 

 nucleus of 



in a shell composed of two valves, which are united along the 

 back, whilst they are free along their lower margin, where they 

 separate for the protrusion of a large and strong anterior pair of 

 prehensile limbs provided with an adhesive sucker and hooks, 

 and of six pairs of posterior legs adapted for swimming. This 

 bivalve shell, with the members of both kinds, is subsequently 

 thrown off; the animal then attaches itself by its head, a portion 

 of which becomes excessively elongated into the "peduncle" of 

 the Barnacle, whilst in Balanus it expands into a broad disk oi 

 adhesion ; the first thoracic segment sends backwards a prolonga- 

 tion, which arches over the rest of the body so as completely to 

 enclose it, and of which the exterior layer is consolidated into 

 the " multivalve" shell; whilst from the other thoracic segments 

 are evolved the six pairs of cirrhi, which are long, slender, 

 many-jointed, tendril-like appendages, fringed with delicate fila- 

 ments covered with cilia, whose action serves both to bring food 



