42 



perpendicular to the margin (fig. 29). These apparent 

 prisms are, of course, the obHquely cut edges of the 

 laminae. 



The shell is everywhere penetrated by very fine tubes 

 and irregular channels. These are more apparent in the 

 vertical section taken perpendicular to the shell margin, 

 where they seem to be cut, for the most part, transversely. 

 In a section at right angles to this they are by no means 

 so obvious. It is evident from this that the black appear- 

 ance of these cavities is due not so much to pigment, as 

 Ehrenbaum supposes, as to the air included in the process 

 of preparation of the section. They are very regular, 

 following the planes of deposition of the shell laminoe. 



The periostracum {Epic, fig. 29) appears in section, not 

 as a regular layer on the external surface, but rather in 

 irregular fragments and patches. 



General Organisation, Mantle and Foot. 



In the ordinary cockle not preserved by any special 

 method, the animal is completely retracted within the 

 shell. The shell margins fit together very closely except 

 at the posterior extremity where, in the relaxed condition, 

 the siphons protrude. Even here on account of the con- 

 traction of the siphonal tubes, the niantle cavity is 

 completely closed. In this condition it is difficult or 

 impossible to open the shell without injury to the soft 

 parts, and, when this is done, the animal is so much 

 distorted owing to muscular contraction that the true 

 relations of the parts are not evident. The animal is best 

 prepared for dissection either by gradually adding a 4% 

 solution of cocaine to the water in which the expanded 

 animal lies, or by placing it at once in a 1% solution, and 

 allowing it to extend, which generally happens in the 

 course of an hour. The irritability of the siphons and 



