372 



ARTHROPODA 



(fig. 390) is drawn out in the Lepadida? into a long muscular stalk (fig. 115). 

 To this attached life are related all the peculiarities of structure. A fixed 

 animal has greater need of protection than one which can flee from its 

 enemies, therefore we find right and left mantle folds capable of com- 

 plete closure, like those of an ostracode, each with two calcified plates 

 the scuta and terga (figs. 115, 390, fy 

 s, /), the first cephalic, the other pos- 

 terior, in position. Between the 

 pairs of these is the gap through cr 

 which the feet are protruded. 



Besides there are other calcified por- 

 tions, one of which, the carina (r), 

 corresponds to the hinge-line of the 

 ostracode and in some Lepads is sup- 

 plemented by a farther unpaired piece, 



a -. 



t . 



.- p 



- c 



FIG. 390. FIG. 391. 



FIG. 390. Balanits hameri* acorn barnacle (from Lang, after Darwin). Formed of 

 rostrum, lateralia, and carina, the operculum of scuta (s) and terga (/). 



FIG. 391. Anatomy of Lepas (after Claus). a, adductor muscle; c, carina; 

 cr, cirri (feet); g, cement gland; I, liver; o, o', ovary and oviduct; p, penis; t, testis; 

 tr, tergum; v, vas deferens. 



the rostrum. In the Balanidae the rostrum and carina are much stronger, 

 while between them other paired pieces, the lateralia, are intercalated. Later- 

 alia, 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. 390). 



The body in both lepads and balanids has essentially the same struc- 

 ture. 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 abdomen 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 



