438 THE ZOOLOGIST. 



sary for the respiration is drawn, and this water is ejected through 

 the other. Through another opening in the mantle a short, thick, 

 fleshy foot protrudes. 



The Shell. — The shell is equivalve, gaping at both ends, swelling 

 below, thin ; transparent and white in appearance. It differs notably 

 from most acephalous molluscs, for there are certain accessory 

 pieces, smaller than the true valves, and placed near the hinges ; 

 these extra parts being connected in some way with the creature's 

 peculiar mode of life. In addition to differences of structure, there 

 are also differences in the use of the valves as compared with what is 

 observed in such a form as the clam. When a clam is disturbed it 

 contracts, completely closing the shell. In the case of Pholas the 

 valves must remain open to make room for the siphons. There is no 

 external difference between the male and female, and the eggs are 

 fertilized in the water after being ejected by the female. 



Spaivning. — Pholas spawns in July, for some of them about the 

 end of June were not ripe. As the tides were unsuitable, it was not 

 until July 27th that a further examination showed ripe eggs and ripe 

 sperms, and some eggs were fertilized by taking a few ripe specimens 

 of Pholas, cutting them in pieces, putting them in water, and letting 

 them stand for a short time. The separate fragments were then 

 placed under running water until it was clear, when the eggs were 

 transferred to clean water for examination. As the development of 

 this type of mollusc is well known, it is unnecessary to describe it. 

 The egg is surrounded by a thick dark brown membrane, within 

 which there is a thin clear white layer, internal to which is the 

 yellowish granular protoplasm. Within the protoplasm is a large 

 nucleus, and within the nucleus a small body (nucleolus). The larvffi, 

 unlike the parent, are free swimming. How they become attached to 

 the rock and commence boring has not been observed. The smallest 

 example found was an eighth of an inch long, and had already pene- 

 trated half an inch into the shale. 



Boring. — Each Pholas lives in its hole ; they may be very close 

 together, and the divisions between them may be as thin as paper. 

 Only once have I found two in one hole or burrow. The hole is 

 narrowest at the top, is egg-shaped at the bottom, the greatest width 

 being a short distance above the base. How the creature bores is not 

 exactly known, though various views are current. The different 

 methods suggested may be grouped as mechanical, chemical, and 

 electrical. Mechanical means seems to me the most likely, but 

 whether the shell only is used, or the foot alone, or both together, is 

 not easy to determine. The fact that the widest part of the hole is 



