3 



GENERAL DESCRIPTION. 



The body of the whelk is divisible into three obvious 

 external regions, head, foot, and visceral mass (PL I, 

 fig. 7). A large part of the animal can be extended 

 beyond the mouth of the shell, but the visceral mass 

 always remains hidden, and the entire animal can be 

 retracted when disturbed. The integument of the 

 visceral mass is produced to form that characteristic 

 molluscan structure — the mantle (PL I, fig. 7, Pall.). 

 The mantle forms a continuous cloak round the body, its 

 free edge being just visible at the shell mouth when the 

 animal is extended. It encloses a space, the mantle 

 cavity, which is best developed on the dorsal and anterior 

 surface. The shell is secreted chiefly by the epithelium 

 of the mantle, particularly of the mantle edge. The 

 organs in the mantle cavity will be referred to later. 



The head of the whelk in an extended condition 

 bears anteriorly two appendages, the tentacles (fig. 7, 

 Tent.). These are compressed dorsoventrally at their 

 base but are produced to a fine conical lip. They are 

 capable of considerable extension and contraction, but 

 cannot be introverted. At the base of the tentacles and 

 on their outer sides are a pair of cephalic eyes, situated 

 on slight lateral prominences.* 



Below the tentacles and in the middle line is a 

 conspicuous opening, an apparent mouth. This is, how- 

 ever, not the true entrance to the buccal cavity. The 

 latter opens at the extremity of a retractile snout but has 

 been carried backwards, owing to an ingrowth of integu- 

 ment, and consequently the true mouth is only seen when 



* One specimen of Buccinum undatum found at Port Erin possessed 

 three tentacles — perfectly normal in shape and each with an eye at 

 the base. From the position it is probable that a second tentacle and 

 eye was present on the left side. A similar case in Patella vulgata 

 has been recorded by Bateson. 



