350 Professor Huxley [May 11, 



of different character, the outer brownish, with a friable prismatic 

 structure, the inner dense and nacreous. In the larva, there is no 

 such distinction, and the whole shell consists of a glassy substance 

 devoid of any definite structure. 



The hinge line answers, as in the adult, to the dorsal side of the 

 body. On the opposite, or ventral side, the wide mouth (m.) and the 

 minute vent {an.) are seen at no great distance from one another. 

 Projecting from the front part of the aperture of the shell there is a 

 sort of outgrowth of the integument of what we may call the back of 

 the neck, into a large oval thick-rimmed disk termed the velum («;.), 

 the middle of which presents a more or less marked convex promi- 

 nence. The rim of the disk is lined with long vibratile cilia, and it 

 is the lashing of these cilia which propels the animal, and, in the 

 absence of gills, probably subserves respiration. The funnel-shaped 

 mouth has no palps ; it leads into a wide gullet and this into a 

 capacious stomach. A sac-like process of the stomach on each side 

 {rl., LI.) represents the " liver." The narrow intestine is already 

 partially coiled on itself, and this is the only departure from perfect 

 bilateral symmetry in the whole body of the animal. The alimen- 

 tary canal is lined throughout with ciliated cells, and the vibration of 

 these cilia is the means by which the minute bodies which serve the 

 larva for food are drawn into the digestive cavity. 



There are two pairs of delicate longitudinal muscles (r.s., r.i.) which 

 are competent to draw back the ciliated velum into the cavity of the 

 shell, when the animal at once sinks. The complete closure of the 

 valves is effected, as in the adult, by an adductor muscle, the fibres of 

 which pass from one valve to the other (Fig. 3, a. add.). But it is 

 a very curious circumstance that this adductor muscle is not the same 

 as that which exists in the adult. It lies, in fact, in the fore part of 

 the body, and on the dorsal side of the alimentary canal. The great 

 muscle of the adult, on the other hand, lies on the ventral side of 

 the alimentary canal and in the hinder part of the body. And as 

 the muscles, respectively, lie on ojoposite sides of the alimentary 

 canal, that of the adult cannot be that of the larva which has merely 

 shifted its position ; for, in order to get from one side of the alimen- 

 tary canal to the other, it must needs cut through that organ. But, 

 as in the adult, no adductor muscle is discoverable in the position 

 occupied by that of the larva, or anywhere on the dorsal side of the 

 alimentary canal ; while, on the other hand, there is no trace of any 

 adductor on the ventral side, in the larva — it follows that the dorsal 

 or anterior adductor of the larva must vanish in the course of develop- 

 ment, and that a new ventral or posterior adductor must be developed 

 to play the same part and replace the original muscle functionally, 

 though not morphologically.* 



The larva of the cockle has, at first, like the oyster larva, only one ad- 

 ductor, which answers to the anterior of the two adductors which the cockle 

 possesses in the adult state. 



