of the Shell and Tube 0/ Aspergillum. 425 



it, through which the foot of the animal is emitted in its free 

 state, and as the cap is moulded on the mantle and the processes, 

 there is found in the middle of the cap a narrow slit, with the 

 inner edge raised up, opposite to this pedal opening in the 

 mantle. To show how accurately the cap is cast on the surface 

 of the swollen mantle, there may be observed on its outer sur- 

 face near the slit, the wrinkles which were to be seen on the 

 skin of the mantle converging towards the pedal aperture. 



The umbo of the valves, showing the form of the shell of the 

 very young animal, are to be seen on the outer surface of the 

 lower part of the tube ; and the plate, consisting of the united 

 valves, is to be seen on the inner surface of the tube, with the 

 well-marked submargiual muscular scars nearly circumscribing 

 the shape of the very gaping valves. 



The upper end of the united siphons often repairs any break 

 that may occur at the smaller end of the tube. In some 

 species, the animal expands the edge of the apical portion 

 of the siphon, and the edge of the shelly tube of the animal is 

 expanded into a shelly frill for its protection. Sometimes these 

 expansions of the end of the siphons take place periodically, and 

 a shelly frill is formed for its protection each time ; then the 

 end of the tube is furnished with a succession of crumpled shelly 

 frills or ruffles at a certain distance apart from each other. 



Thus the development of the shell and tube of this animal 

 may be divided into four states : — 



1 . The animal, with its two shelly valves, living free in the sea. 



2. The animal takes up its abode in the sand, living free in 

 the shelly tube it has formed. 



3. The two valves of the shell become united into a single 

 plate, which is formed into part of the base of the tube still open 

 below. 



4. The fringe, and then the perforated cap, closing the base 

 of the tube, are formed. 



The ClavagellcB and allied genera go through all these stages, 

 like Aspergillum, except that the valves never become united, and 

 only one of them is imbedded in and forms part of the tubular 

 house of the mollusk, and it is generally more or less affixed to 

 the cavity in which it is formed. 



These genera, in their first stage, are normal Conehifers; in 

 the second, they resemble the usual genera, which, like Gastro- 

 chcma, live in tubes ; it is only in the third and fourth stages 

 that they develope their peculiar generic characters. 



These mollusks, like the Sti'ombs, Cypi'(2a, and several other 

 genera, when they have once developed their full expansion, do 

 not repeat the process, as is the case with the Murices and many 



