TUIO ANATOMY OF MRGALATltACTUS — KK.SIKVICX. 425 



In M. araaniis what is hero termed the proboscis-shealli is the 

 homologue of the proboscis-sheath as generally found, i.e., the 

 permaaently iavaginated portion, though it is not produced into 

 a pseudo-snout, Ijut lies wholly in the body cavity. The "pro- 

 boscis-sac" of this mollusc is the homologue of the evaginable 

 and invaginable portion of the typical proboscis. Whether or 

 not it is evaginable cannot be said with certainty, but it seems 

 probable that the posterior half is turned into the anterior half. 

 That it is evaginated so far as to lie within the sheath seems 

 unlikely, in view of its large size, as compared with the cavity of 

 the sheath. The portion here termed "proboscis" is therefore 

 obviously the homologue of the non-invaginable, but protrusible 

 portion of the tj'pical proboscis. 



The moiUh is situated at the anterior end of the proboscis ; it 

 is small for so large a mollusc, and is not armed with jaws. 



The odontophore is large and occupies the greater part of the 

 buccal cavity. 



The raclula is short, and the rad alar- sac, which is straight, 

 lies below the oesophagus. The teeth on that portion of the 

 radula contained in the sac are on the surface directed ven- 

 trally; as the radula enters the buccal cavity it is turned back, 

 so that the teeth come to lie uppermost. 



The dental formula is 1, 1, 1 (PI. xlii., fig. 3). The rachidian is 

 tricuspid, all three cusps close together in the centre of the base, 

 the central cusp largest, the other two much smaller. The 

 base of attachment is broad, but not particularly deep. The 

 laterals are unicuspid ; the cusp is a long, bent blade attached to 

 the inner margin of the broad base of attachment. This denti- 

 tion has been already very briefly described by Macdonald.* 



The oesophagus [ant. ces.) originates from the dorsal surface of 

 the pharynx or buccal cavity and proceeds directly backwards 

 through the proboscis ; it is a remarkably small tube for so large 

 a mollusc, in fact the smallness of the cavity of the whole tract 

 is particularly noticeable ; it passes out of the proboscis and 

 through the wall of the sac, where the two are attached ; thence 

 it turns sharply forward attached to the outer wall of the sac, on 

 the left side thereof, by tine strands of connective tissue ; at the 

 anterior end of the sac it bends down and passes forward along 

 the floor of the cavity towards the anterior end ; its final position 

 here was lost in the general decay. It was found on the left 

 side of the cavity passing backwards {post, ces.) along the floor; 

 after passing out of the body cavity its course is through the 

 pericardium, thence along the axis of the coil to the stomach. 



The stomach (stom.) (PI. xxxix., fig. 2) is tubular, its lumen not 

 ver}' much greater than that of the posterior portion of the cesopha- 



* Macdonald— Ann. Mag. Nat. Hist., (4), i i.,]86S, p. '24.3. 



