336 



ARTHROPODA. 



which is termed the " paries " (fig. 197, B,^>), which is attached 

 by its base to the " basis " of the shell. The " paries " grows 

 downwards, so that the whole shell increases by additions 

 made 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 " alee," according 

 to their shape (fig. 197, B, c). Sometimes the paries has a 

 " radius " on both sides, sometimes " alse " 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 



Fig. 197. Shell of Balanid<6. A, Diagram of the shell of Balanus : I, I, 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, Compart- 

 ment 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.) 



limbs, is called the "carina" (fig. 197, A); and the com- 

 partment immediately opposite to this " 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. 197, 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 



