272 MR T. J. EVANS ON 



THE ALIMENTARY SYSTEM. 



It will be seen in fig. 2 that the general plan of the alimentary system of Bathydoris 

 resembles that of the typical Doric!, but a detailed examination shows that there are 

 very important differences. The great size of the buccal mass (b.m.) was emphasised 

 by BERGH as a characteristic feature of the genus. Indeed, BERGH claimed its supposed 

 relationship with the Tritonids solely on the character of the buccal mass and the radula. 

 The mouth is surrounded by two sets of lips, an outer and an inner. The outer lips are 

 crinkled and fleshy, and leave a wide gape into which the inner lips project. These are 

 merely the thickened rim of the outer integument limiting the buccal opening. The 

 inner lips (p.m., figs. 2 and 3) are a pair of lateral pads enclosing a relatively small open- 

 ing leading into the buccal cavity. The pads have the consistency and appearance 

 of hyaline cartilage Histological examination shows that the hyaline substance is a 

 cuticular secretion which is continued in varying thickness as a lining of the alimentary 

 tube from the buccal rim to the stomach. The underlying epithelium consists through- 

 out of tall slender columnar cells, and corresponding columns of the secreted matter are 

 faintly visible in the cuticle. 



The buccal cavity contains a pair of dark-brown horny jaws (J., fig. 3) supported on 

 muscular pads (P.J., fig. 3) which separate them from the globular odontophore (o.) 

 occupying the middle of the floor. The free edges of the jaws are quite blunt, and each 

 is produced into a slight beak opposite the mouth ; so that they are probably used as 

 a prehensile organ. The mouth must be capable of far more extension than might be 

 supposed from its preserved state in order to bring the jaws into action. The radula 

 (R.) is narrow in front and broad behind where it enters the radula sac, and the sac is 

 entirely contained in the substance of the odontophoral mass. The first row of teeth has 

 three teeth on either side of the rhachidian tooth and the radula broadens to 90. 1. 90, 

 the formula of the youngest row. The total number of rows is about 50. The rhachidian 

 tooth (Rh., fig. 4) has a broad base on which stands a backwardly directed cusp. The 

 laterals (L. 1 to L. v ) are formed on the same plan, but the cusp bends from the base 

 towards the middle line. The first four laterals differ from the rest in having shorter 

 and blunter cusps. Unlike those of the other species, the extreme laterals show little 

 sign of reduction. The teeth are firmly fixed in the specially thick cuticular covering 

 of the radula mass, which is itself bilobed with a deep median depression. The direction 

 of the cusps and of the underlying muscles suggests that the radula is used for 

 gripping the food during trituration. The approximation of the cusps of the first 

 laterals thus entailed would also explain the broken and irregular appearance of the 

 rhachidian cusps as well as the reduction in length of the cusps of the first few laterals. 

 Comparison with the figures published by BERGH and ELIOT shows that the present 

 species can be identified by its radula alone. 



The (esophagus (ce. and cr., fig. 2) seems to differ from that of the other species in 

 that it turns to the left, even at its origin from the buccal mass. It is a broad, sigmoid 



(ROY. soc. KDIN. TRA.NS., VOL. L., 194.) 



