12 



those authors who have considered the operculum 

 homologous with the other valve of the lamellibranch 

 shell ; and with the retractor muscles of the foot, by those 

 who consider the operculum as representing the byssus 

 of the lamellibranchs. Both theories are untenable. 

 The operculum is probably a new structure, and the 

 search for homologies is sometimes carried too far. 



The foot exhibits very great changes in size, 

 according to whether it is fully expanded or completely 

 contracted. Whilst the whelk is creeping about, the foot 

 is almost constantly changing in shape, and if not in total 

 volume the size of different regions at least varies. This 

 power of change is due to the vascular supply and the 

 muscular structure. The pedal arteries form a very 

 complete network extending throughout the foot. Blood 

 runs back through a large central sinus. The expansion 

 takes place through the forcible intrusion of blood into 

 the foot. If the foot of an expanded animal be suddenly 

 cut off with a very sharp scalpel, the sudden outflow, or 

 jet of blood, seen as one severs the sinus, is most striking. 

 This is due to the muscular contraction which begins 

 immediately the foot is touched with the knife. The 

 blood, in fact, has most important functions to perform 

 in effecting both the expansion of the foot and the 

 extrusion of the proboscis. 



The creeping action of the gastropod foot has already 

 attracted considerable attention. In most cases when the 

 animal is in motion a series of waves can be seen coursing 

 along the foot. These may be in a direction from behind 

 forwards — direct (Aplysia, Doris, etc..) or in the reverse 

 direction anterior to posterior— retrograde (Littorina 

 littorea and others). In some gastropods again the foot 

 is divided into two longitudinal halves and these move 

 alternately, both exhibiting systems of waves. In 



