112 



THE CRUSTACEA 



Generally included with the Operculata, though possibly deserving 

 to rank as a separate sub-order, is the curious little group of Asym- 

 metrica, comprising the single genus Verruca. These are sessile, 

 like the true Operculata, and have the shell composed of a wall 

 and closed by a movable operculum. But the operculum consists 

 of the scutum and tergum of one side only, those of the other side 

 being fused to form one-half of the wall, which is completed, on 

 the side of the movable opercular plates, by the greatly developed 

 and displaced rostrum and carina. 



FIG. 64. 



Diagrammatic vertical section of Balanus. The cirri are cut short. A, anus ; ant, 

 antennule; B, mouth; C, carina; c, cerebral ganglion; cap, lamellae of the "wall"; 

 ci, cirri; c.p, parietal canal; c.pal, mantle - cavity ; c.r, radial canal of the base; c.t, 

 testicular caeca; E, stomach; gl.ce, cement-glands; i, intestine; in/, "infundibulum" (con- 

 taining a prolongation of the mantle) ; l.ext, external lamina of the wall ; m.a.s, adductor 

 scutorum muscle; m.d.s, depressor scuti muscle; m.d.t, depressor tergi muscle; , ventral 

 nerve-mass ; o, opening of mantle-cavity ; ov, ovary ; p, penis ; R, rostrum ; S, scutum ; s.oe, 

 egg-mass in mantle-cavity ; s.ro, rostral blood-sinus ; T, tergum ; v.s, seminal vesicle. (From 

 Gravel's Monographic.) 



In the Pedunculata the shell is formed of simple calcined plates, 

 increasing in thickness by the application of successive layers on 

 the inner surface, while the uncalcified cuticle between them scales 

 off periodically to admit of growth, with the formation of a new 

 cuticle underneath. In the simpler Operculata (Ckthamalus), the 

 compartments forming the wall are of this nature, but in most of 

 the genera composing this group they have a more complex 

 structure, being traversed by canals running parallel with the 

 surface and containing prolongations of the hypodermis (Fig. 64, 

 c.p and wf). The complexity of the shell reaches its maximum 

 in the genus Coronula, the species of which attach themselves to 

 the skin of whales. In these, the folding of the wall gives rise to 



