ANATOMY AND LIFE HISTORY OF MOLLUSCA. 165 



insect's head, and the large nerve-ganglion in the substance of the 

 shell as the cerebral ganglion of the species'? It seems to me that 

 the shell in this case should be regarded as a brain-case, as well 

 as a box for the enclosure of the soft tissues of the animal, and 

 thus a new office for the shells of Mollusca comes into view. Most 

 persons will say at once that this case may be quite exceptional, 

 but I will deal with that suggestion presently. 



The cerebral ganglion, as I must be allowed to call it, in the 

 substance of the shell, has already been described in speaking of 

 the nervous tissue. It may be seen, I think even on the outside 

 of the shell, where it is marked by certain dark lines of pigment. 

 In Trigonia lamarckii, Gray, these may be traced in the form of 

 double lines of darker colouring matter corresponding with the 

 former lines of growth. They are equally visible on the outside 

 and the inside of the shell. So here is an explanation of the 

 meaning of these dark bands of colour, though the reason for them is 

 not so readily assigned. The nerve-sheaths connected with these 

 ganglia pass round the shell to the anterior adductor muscle whence 

 they issue into the soft parts of the animal. Here they may be 

 seen in section almost like the pipes of an organ, and larger than 

 any nerve-strand in the soft parts as far as my observation extends. 



Now with the evidence before me of the small size of the ganglia 

 in the soft parts of Trigonia I am inclined to regard the ganglia 

 in the shell as a real cerebral centre from which the whole body 

 is supplied, and the order which the nervous fibres take is from 

 this centre, to the eyes on one side, and the soft parts on the other. 



But these Mollusks have hitherto been classed amongst the 

 acephalous or headless Mollusks, and yet we see that such a 

 description is entirely out of place. To say nothing of the eyes, 

 the cerebral ganglion contained in its shelly brain-case, should 

 lift it quite out of the category of headless animals. Hitherto it 

 has been said and taken for granted without dispute that the 

 relatively feeble development of the cerebral ganglia in the bivalves 

 is due to the absence of a head and the accompanying sensory 

 organs of a head. But what is entitled to the name of a head, if 

 not these valves provided unmistakably with a large nervous 

 centre and so richly endowed with sensory organs. 



But then the question arises, how far is this structure in the 

 case of one species of Trigonia exceptional 1 Are the other 

 members of the genus similarly endowed, or are there other genera 

 to which the same characters are to be ascribed 1 That Trigonia 

 lamarckii,Gray, is a richly endowed shell, there cannot be any doubt. 

 No shell that I have met with as yet has the eyes and the eye- 

 facets so beautifully marked, but I think I have seen something 

 nearly equal to it amongst Pectens. No shell that I have met 

 with as yet presents such beautiful and easily accessible ganglia. 



