of Spondylus varius. 411 



The laminae, which are deposited in a continuous series 

 of superimposed layers at the hinge of the lower valve, are 

 not continued in a like state of superposition throughout ; 

 they soon separate from each other, and do not again unite 

 except at the space corresponding to the adductor muscle, 

 and at the circumference of the valve. 



The interspaces of these successive layers of the growing 

 Spondylus cannot, from the absence of a medium of intercom- 

 munication, serve any purpose hydrostatically with reference 

 to locomotion : it is a singular fact, indeed, that the structure 

 above described is not constant in the Spondylus varius. I 

 am informed by Mr. Samuel Stutchbury that it occurs only in 

 those individuals which happen to have been developed be- 

 neath the overhanging ledge of a coral reef, or in situations 

 where they would be liable to be overgrown by coral if they 

 had not possessed the power, like the Vermeti and Magili, 

 of keeping their respiratoiy and nutritive apertures on a level 

 with the surrounding zoophytes. A glance at the figure will 

 show at once the process by which the Spondylus has, under 

 these conditions, gained the necessary ground. It must be 

 remembered that, like the oyster, it is attached by the exteri- 

 or surface of the lower valve to some extraneous body ; it has 

 no power of locomotion ; and therefore, in order to bring the 

 margins of its mantle to the influence of light, and of the free 

 currents of sea water, it is endowed with the power of carry- 

 ing forwards its dwelling-chamber by a series of new forma- 

 tions. These are not, however, to be regarded as the results 

 of a purely voluntary act, but to the inevitable obedience of 

 the secreting organs of the shell, (as long as the animal has 

 strength and material left), to the stimulus arising from im- 

 perfect insolation, respiration, and nutrition. The septa of 

 the deserted chambers we can of course only view in the light 

 of mere dermal exuviae still left adhering to the animal, to 

 which, being deprived of locomotion, they can be no incum- 

 brance. It is highly probable that all the chambers are ori- 

 ginally filled with fluid, as more or less is found in the outer 

 ones of the specimens brought to this country. 



In the Testaceous Cephalopods a new structure is added to 

 their camerated shell, viz., the siphon, whereby the exuvial 

 layers and the deserted chambers are converted into a hydro- 

 static instrument, subservient to the locomotion of the animal. 



The fluid contained in the large specimen here described, 

 was carefully withdrawn from the outer chambers before the 

 section was made : and my friend Dr. Bostock, who kindly 

 undertook a chemical examination of it, has transmitted to 

 me the following account of the results of his analysis. 



