67 



made at posterior end of the foot, and the water was found to issue at 

 the two anterior lateral pores just behind the base of the lateral pedal 

 tentacles. These experiments were repeated several times on different 

 specimens and the same results obtained each time. After squirting 

 water gently through these anterior and posterior pores the syringe was 

 filled with a 10^ solution of formalin, and this was passed through the 

 pores for some time, in order to fix the opening in an expanded state, 

 the material being at the same time dipped in corrosive sublimate. A 

 second series of sections of anterior and posterior pores was then made 

 in all three planes as before, and it was found that the inpushings of 

 the ectoderm had at their base a definite opening. There is only a short 

 invagination of the ectoderm, opening abruptly into large space in the 

 foot 'Fig. 2 and 3). This foot cavity is bounded by no definite epithelial 

 wall, but by muscular tissue, and there are numerous muscular strands 

 connecting floor and roof, and dividing the cavity up into a number of 

 spaces. The disposition of the muscular strands is readily seen by cutt- 

 ing a thin tangential section of a portion of the foot. These strands 



MU3 



Fig. 2. A section in posterior region of foot showing pores leading into foot cavity. 

 ITP, water pore; E, epithelium; FC, foot cavity; 7, base of tentacle; il/f/j, MU-ii 



ML\ muscle layers. 



are in three groups, one group in the longitudinal plane, and the other 

 two at right angles to one another, and oblicjuely to the longitudinal 

 group, giving the whole a network-hke appearance. It is seen in some 

 of the sections that well marked muscular bands are attached to the 

 inturned epithelium at the pores, and these are probably of the nature 

 of sphincters for closing the pores. 



These aquiferous spaces are completely separated from the circu- 

 latory apparatus, and when filled with water serve to distend the foot 

 in the action of crawling and burrowing in the sand, while, when the 

 water is out of the foot, it is readily withdrawn into the shell. 



Some of the habits of the animal are noteworthy in this connection. 

 At high tide it remains buried in the sand, but at low tide it comes out 

 to feed, and, in the summer months, large numbers may be seen crawl- 

 ing about in the moist sand. A common article of diet is the polypes 

 of PhysaUa, which are washed up on the beach in immense quantities 



5* 



