MOLLUSC A. 35 



at the posterior end (?/'), are very large and prominent. 

 The foot has a sUght muscular ridge. The peculiar 

 appendage {bg) between it and the mouth is a part of 

 the foot ; and in the hollow in the young are secreted 

 long threads, like those of the common mussel, with 

 which it anchored itself to the rocks until old enough to 

 break away and take care of itself. The palpi are 

 similar to those of other forms, except just about the 

 mouth, where they are changed into the remarkable 

 branching tentacles {tp) shown in the figure. The 

 mantle border is very thick, and has numerous rows of 

 tentacles, especially on the outer part. The eyes are 

 numerous, and between the larger ones are often 

 smaller and more imperfect ones, indicated by black 

 dots. There is a strong muscular border inside {mm), 

 attached closely to the shell all around. The gener- 

 ative organs are situated in the foot, and open into the 

 organs of Bojanus. 



The movement of the animal through the water is 

 accomplished by the quick and forcible shutting of the 

 valves. This drives the water out in a broad jet, which, 

 by reaction, throws the shell with considerable celerity 

 forwards in whatever direction it may happen to be 

 pointing. 



The result of this is in all probability to carry it by 

 an irregular, zigzag line in the direction in which the 

 tide is moving. At least this seems the only reasonable 

 way to account for any definite course being steered 

 by them during their migrations ; so that, by selecting 

 their opportunity, they can move towards shallower 

 water, or away from it, at their pleasure. One placed 

 in an aquarium can be made to swim in this peculiar 



