424 Dr. J. E. Gray on the Development 



When the free animal has found a situation which is suited 

 for it to take up its fixed abode, it buries itself in the sand, and 

 commences the formation of its tube, which it gradually in- 

 creases in size downwards as the body of the animal grows in 

 length and diameter, living free in the tube, as the Gastrochana 

 does in its habitation. The valves, during this period of the life 

 of the mollusk, are extended greatly in width (that is, from the 

 dorsal to the ventral edge), to fit themselves to the increased 

 diameter of the body ; their front edge is rounded, and the 

 hinder edge is truncated, leaving a large gape for the passage of 

 the siphons, which form a thick cone. The animal and shell 

 remain free in the shelly tube imtil it has nearly reached its 

 predestined size, — the actual size being more or less influenced 

 by the abundance or scarcity of food in the locality. 



When the animal has nearly reached maturity, and feels the 

 approach of the period common to all natural beings, it prepares 

 to complete its development, and shows all its peculiarities. And 

 as Murices, Cassides, and other Mollusca, at such times pro- 

 duce the edge of their mantle into ridges or processes, which 

 require the production of the parts of the shell which are called 

 varices, for their protection, so in this genus, at this time, the 

 valves become united together into a single plate by the deposit 

 of calcareous matter on their inner surface ; and this plate is 

 surrounded by and imbedded in the part of the base of the 

 tube that remains to be formed : this causes the animal, which 

 was up to tliis time free to move about as it pleased in the tube, 

 to become so fixed in it, that it is only able to contract and elon- 

 gate the hinder part of its body formed of the united siphons. 



As the process of development proceeds, the mantle, which 

 was simple, is furnished with a series of cylindrical processes 

 round the circumference of its front end, occupying the base of 

 the tube. These processes are at first simple, but as they are 

 extended in length, become forked, and sometimes re-forked; 

 and as they are produced, a shelly tube is deposited round them, 

 for their protection, which is at first simple, but at length be- 

 comes forked, like the cylindrical fleshy processes which they 

 protect ; and it is these small tubes, deposited for the protection 

 of these processes, that form the fringe, as it is called, round 

 the base of the large tube, in which the mollusk resides. At 

 length, when the fringe is formed, the entire surface of the front 

 or lower part of the mantle developes similar regularly dis- 

 posed cylindrical processes, and it is covered with a convex shelly 

 cap, and furnished with small simple tubes or shelly cases, like 

 the tubes of the fringe, for the protection of the processes on 

 the front of the mantle above referred to. 



As this part of the mantle has a small sul)central aperture in 



