424 ARTEROPODA. 



It is clear that a fixed animal has greater need of protection than 

 one which can flee from its enemies, therefore we find not only a 

 right and left mantle and a shell capable of complete closure, 

 like that of an ostracode, but also in this calcified plates, the scuta 

 and terga (figs. 114, 427, s, t), the first cephalic, the other pos- 

 terior, in position. 



Between the pairs of these is the gap through which the feet 

 are protruded. Besides there are other calcified portions, one of 

 which, the carina (fig. 114, c), corresponds to the hinge-line of 

 the ostracode and in some Lepads is supplemented by a farther 

 unpaired piece, the rostrum. In the Balanidae the rostrum and 

 carina are much stronger, while between them other paired pieces, 

 the lateralia, are intercalated. Lateralia, rostrum, and carina arise 

 from a base (usually calcareous) and form a capsule, closed above 

 by a double valve formed of the paired scuta and terga, between 

 which, when open, the animal can be seen (fig. 427). 



The body in both lepads and balanids has essentially the same 

 structure. It is flexed ventrally, so that mouth and vent are near 

 each other, and bears six pairs of feathered feet, or cirri, which, 

 when extended, become widely separated and form a most efficient 

 means of straining small organisms from the water and conveying 

 them to the mouth. These feet are biramous, with their branches 

 ringed and thickly haired. Behind them is a rudimentary abdo- 

 men and an elongate penis; while the mouth is surrounded by a 

 pair of mandibles and two pairs of maxillae. 



In internal structure the most noticeable feature is that the 

 animals, in contrast to almost all other arthropods, are hermaphro- 

 ditic, a condition possibly correlated with their sedentary life and 

 the consequent need of self -impregnation. Yet it is to be remem- 

 bered that the common forms have a long 

 penis, so that these animals, crowded 

 closely together, can fertilize each other. 

 In cases of several species which live 

 solitary complementary males occur. 

 These are very small, purely male forms, 

 with extremely simple structure (fig. 

 428), which live inside the mantle cavity 

 near the genital openings. The un- 



Se ^ mented ^^ ls enclosed in a sac ( a 



lobes; m , muscles ;oc, ocellus: soft-skinned shell), and anchored by the 



p, penis; t, testis; vs, seminal n 



vesicle. antennas. The long penis protrudes from 



the mantle. In the genus Scalpellum there are purely hermaph- 



