498 



CRUSTACEA. 



round the base. The paries of each compartment is flanked by 

 wing-like portions, which differ from the paries in appearance, 

 and are called "radii" and " alae," according to their shape (fig. 

 355, b, c). Sometimes the paries has a "radius" on both sides, 

 sometimes "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. 355, a); and the compartment immediately opposite 

 to this the "rostrum." The remaining compartments are "lateral," 

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

 rostrum " rostro-lateral," and the middle one simply "lateral" (fig. 

 355, a) ; but the three rarely coexist. 



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 



Fig. 355. — Shell of Balanidae. a, Diagram of the shell of Balanus : II, Basis; c, Carina; 

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

 partment, b, Compartment with two radii (r r), flanking the paries (/>). 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 {t). (After Darwin.) 



along the opposite margin. Similarly, each tergum opens and shuts 

 against its fellow along one margin (the " carinal " margin), and 

 articulates with one of the scuta along the opposite margin. The 

 apex of the terga (fig. 355, 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 de- 

 pressor muscles. 



