VI SCAPHOPODA. 



place at the larger end and on the inside of the tube, and at the 

 same time the shell is absorbed at a slower rate from the smaller 

 end. The slits and notches in this end are therefore formed by ab- 

 sorption, being wholly different in genesis from similar structures of 

 the peristome in Gastropods. In some forms (pi. 9, fig. 51 ; pi. 14, 

 figs. 20, 21 ; pi. 18, fig. 4,8) upon the practical cessation of growth 

 and absorption upon the attainment of maturity, there is a supple- 

 mentary tube built out from the edge of the anal orifice. Thi& 

 structure is wholly different from the interrupted "sheath" of very 

 frequent occurrence (pi. 18, fig. 16, 17) which is due to the slower 

 absorption of the dense inner layer of the shell. 



The shell contains a very slight organic basis, leaving no appre- 

 ciable amount when dissolved in acid. It is formed of three distinct 

 layers. The inner layer (hypostracum , pi. 38, fig, 3, h) is composed 

 of long prisms, rounded at the angles and tapering at the ends. 

 This layer is thinnest at the aperture, thickest toward the apex. 

 The thick middle layer {ostracum, pi. 38, fig. 3, o) is built up of 

 short prisms in bundles lying at right angles with each other. The 

 outer layer (periostracum, pi. 38, fig. 3, j)) is thin and wholly struc- 

 tureless. It covers the sculpture, ribs, etc., of the shell. 



The snout or proboscis shows considerable variation in form, as 

 well as in the number and presence of mouth-lobes in the several 

 species and genera. At its base, there are two plate-like folds of the 

 integument, which bear numerous, long, extensile filaments, each 

 terminating in a spoon-shaped expansion. These " captacula " are 

 prehensile, catching foraminifera, etc., upon which the Scaphopod 

 feeds (pi. 38, fig. 6, one captacle greatly enlarged). 



The foot is inserted immediately adjacent to the head, ventrally. 

 In Dentalium the foot is pointed, with a circular "epipodial" ridge, 

 interrupted dorsally, some distance from the end, which gives it a 

 trifid or fleur-de-lis shape. In the Siphonodentaliidce the " epipo- 

 dium " is subterminal, not interrupted dorsally, and forms a disk 

 with crenate edge, with or without a central filament representing 

 the conic point of the Dentalium foot. It would seem that this ex- 

 panded disk (as shown on pi. 24) is capable of contraction to a 

 slender conic form (pi. 26, fig. 79) ; burrowing being effected as in 

 Solen, etc., by thrusting the conic foot downward, then expanding 

 it distally for an anchor, and pulling the shell down by contraction 

 of the foot retractor ; the process being then repeated. 



The gut (pi. 38, figs. 4, 5) is short, closely convoluted. Liver (pi. 

 38, fig. 1,2,/) large, lying along the ventral side. 



