140 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



of minute eyes in the shells in immense numbers is a characteristic 

 of many Mollusca. Like every other arrangement of this kind 

 in nature, it is subject to much variation. It is abundantly 

 present or nearly absent, it is larger or smaller, prominent or 

 inconspicuous in different groups. There is sometimes a 

 geographical association of certain forms of these organs, 

 dependent upon the conditions of life being the same, such as 

 the climate, temperature, food, and so forth. 



The Siphonostomata have not been subjected to such 

 examination as the plant-feeding littoral shells ; but the 

 Tritonidse and Buccinidae have furnished abundant instances, 

 amongst which I may name Triton spengleri. Lam., Buccinum 

 alveolatum, Kiener, and Polytropa margine-alba, Fisch., of the 

 family of Purpuridse. In most of the above-mentioned families the 

 lines of growth are represented by fine imbrications of shelly 

 matter, often forming varices on the spiral ribs. At the junction 

 of the shelly imbrications with these ribs, there are generally 

 sense-organs, which follow one another in regular progression to 

 the mouth of the shell. 



In giving illustrations of the way in which these sense-organs 

 occur, a few species only can be dealt with, because the discovery 

 is too recent to have allowed time for minute dissection in a great 

 many cases. I must confine myself to these few, and will 

 continue with the simple, conical, littoral shell with which this 

 Essay was commenced, namely, Patella tramoserica. 



We find, generally, that a very uniform system of colour 

 prevails amongst the limpets. Radiating lines of brown and yellow 

 spots and angular markings occur very commonly. Beautiful lines 

 of rich deep brown, golden yellow and orange are common among 

 a good many species. What purposes they subserve cannot be 

 said ; but that they do belong to some very important economy 

 we may be convinced, for there are certain coloured dots on the 

 edges of the mantle which correspond with the dark coloured rays 

 on the shell. It has occurred to me that perhaps these bands of 

 colour in the shell may serve as pigment-coats for some of the eyes 

 in the shell ; though it would be difficult to explain in that case 

 why they should be absent from places where the organs are just 

 as numerous. A little farther explanation about the anatomy of 

 the species of limpet with which we are dealing will serve to clear 

 up some matters connected with the eyes or other sense-organs. 



I find that in this species of Patella, the mantle stretches down 

 to the margin of the upper plate already spoken of. I distinguish 

 three folds of tissue : namely — the shell or testaceous fold which 

 lies next to the shell, the tentacle-bearing or median fold and the 

 branchial fold. All these extend between the foot and the shell, 

 excepting for a space round the head, where the branchial fold 



