70 IJMIFOSSOR TAI.rOIDEUS. 



lar organ docs not exist and yet the fact that in the mid line tlie cells are more 

 than usually high and slender and arc in close proximity to nerves from the labio- 

 buccal ganglia indicates that the area exercises a sensory function. The radula 

 and its supports and attendant musculature are enormously developed and indi- 

 cate active predatory habits, but in every case the alimentary canal contains little 

 besides a few diatoms, sponge spicules, and a small quantity of inorganic detritus. 

 The radula with twenty-eight transverse rows is of the distichous type (Plate 34, 

 figs. 3, 6), the long claw-like teeth being united while in the radula sheath by a 

 clearly defined basement membrane. When freely exposed this membrane 

 splits along the mid line and the teeth become located on each side of a deep 

 cleft in the forward end of the radular supports (Plate 10, fig. 10). Odonto- 

 blasts, in typical fashion, form the teeth which are subsequently enveloped by 

 numerous enamel cells. 



The radular supports comprise two great masses of muscle and connective 

 tissue which together form an ovoid mass grooved dorsally to hold the radula 

 tube. To these numerous muscles attach that are in part responsible for the 

 movements of the teeth. A detailed description of these and other muscle 

 bands has been given in another place (Heath '05) and an attempt has been made 

 to determine their functions. 



The stomach is sharply differentiated from the remainder of the digestive 

 tract (Plate 10, fig. 4) and occupies practically all of the space between the end 

 of the radular supports and the forward border of the gonad and digestive gland. 

 Its epithelial lining is produced into a number of heavy folds that gradually 

 blend with those of the oesophagus. In most cases the intestine leaves the pos- 

 terior end of the stomach in the mid line, and immediately ventral to this union 

 the liver opens by a single pore. This last named organ is relatively voluminous, 

 filling much of the space beneath the gonad between the stomach and forward 

 cloacal wall where it ends blindl>'. The intestine, of practically the same calibre 

 throughout, makes its way by a fairly direct route to the front end of the peri- 

 cardium. Here it bends abruptly downwartl and jxissing under the cloacal wall 

 opens to the exterior in the mid line. 



A clearly defined connective-tissue septum bounds the head cavity pos- 

 teriorly as in the Chitons. It passes immediately behind the radular supports 

 and is penetrated by the alimentary canal, dorsal aorta, and pedal sinus. 



The pericardial cavity is of trihedral form and encloses a tubular and more 

 than usually muscular heart without any distinct subdivisions. The aorta 

 passes out from its forward border, and as a distinct tube with definite walls 



