CRUSTACEA. 1 5 I 



" alae " on both sides, and sometimes an ala on one side and a 

 radius on the other. 



The separate 'compartments of the shell receive special 

 names according to their position. The compartment at the 

 end of the shell where the animal thrusts out its cirrated limbs, 

 is called the " carina" (fig. 96, A) ; and the compartment im- 

 mediately opposite to this " rostrum." The remaining com- 

 partments are " lateral," the one nearest the carina " carino- 

 lateral," the one nearest the rostrum " rostro-lateral," and the 

 middle one simply " lateral" (fig. 96, A); but the three rarely 

 coexist. 



Fig. 96. Shell of Balanidae. A, Diagram of the shell of Balanns ; /, / Basis ; c Carina ; 

 k Rostrum ; m Rostro-lateral compartment ; n Lateral compartment ; o Carino-lateral 

 compartment. B, Compartment with two radii (r, r) flanking the paries (p). C, 

 Compartment with a radius (r) on one side, and an ala (a) on the other side of the paries. 

 D, Internal view of the scutum. E, Internal view of the tergum, showing the spur (s) 

 and the beak (/). (After Darwin.) 



The " operculum " or lid of the shell consists of two pairs of 

 valves, known as the " scuta " and " terga," forming a little 

 pyramid or cone, attached within the orifice of the shell by a 

 membrane. Each scutum opens and shuts against its fellow 

 along one margin (the " occludent " margin), and articulates 

 with one of the terga along the opposite margin. Similarly, 

 each tergum opens and shuts against its fellow along one mar- 

 gin (the "carinal" margin), and articulates with one of the 

 scuta along the opposite margin. The apex of the terga (fig. 

 96, E) often forms a prominent beak, and the basal margin is 

 furnished with a process or " spur." The scuta and terga are 

 not only movable, but are furnished with proper depressor 

 muscles. 



