ANATOMY AND LIFE HISTORY OF MOLLUSCA. 139 



With regard to the land and freshwater shells, observations 

 have still to be made, though I incline to the opinion that these 

 will be found to furnish instances such as I have described. In 

 the case of shells covered with periostraca, the land shells especially, 

 and such species as Triton spengleri, I have not been able to find 

 these visual organs, and this is also the case with some species of 

 Chiton. I do not find that the Chitonidse are more bountifully 

 provided with eyes than other genera, in fact they are less so, and 

 as for Onchidiu?n, the genus in which these wonderful sense-organs 

 were first discovered, the eyes in all the species are proportionately 

 very few. We cannot moreover be sure from the absence of eyes 

 in one particular case, that such a peculiarity is specific, because 

 in some species of Onchidium I have found no eyes in one 

 individual of a species and the full complement in another. The 

 greatest number of eyes in any one valve of a shell is in the case 

 of Trigonia lamarckii, as far as my observations go, and in this 

 case I think there must be about 12,000 in each valve of an 

 adult specimen. 



Those shells which have a smooth or enamelled tegmentum are 

 those in which the eyes are the least numerous, and I would 

 venture to suggest that this may be made up by some special 

 arrangement in the soft tissues of the animal. Thus in Cyprcea and 

 the cowries generally, all who have seen the living animal will 

 remember the number of papilla? of extraordinary shape with which 

 the inner surface of the mantle is studded. These may possibly 

 be sense organs or something to make up for the optical 

 arrangements which are most probably absent from the shell. 



Before entering into any detail about the sense-organs in 

 different species, I may sum up briefly by saying that the presence 



into a concave layer, he says : " Influential in the development of this 

 arrangement is the multiplication of the perceptive elements, and the 

 formation of refracting media. Just as the eyes are completely wanting 

 in the majority of the Scoleina which live in the dark, so also these 

 organs undergo degeneration in the Tubicola among the Chaetopoda. 

 The eyes which are present in the larvae and even in later stages, 

 disappear, or are represented by mere pigment-spots, when they enter 

 on the fixed mode of life. The development of the visual organs on the 

 branchial tufts of the head is an adaption of another kind, which is seen 

 in certain Sabellidae (Branchiomma) ,• in them the eyes are either placed 

 in large numbers on the pinnate branches of the branchial filaments, or 

 at their ends only. In other Annelides there is a similar change of 

 position as compared with the primitive one. In many there are eyes at 

 the posterior end of the body, as well as on the cephalic segment ; and 

 finally in the genus Polyophthalmus there is a pair of eyes on each 

 metamere, in addition to those on the head. We here find an arrangement 

 which is not only of importance as bearing on the estimation of the 

 metameres, but is also a proof that visual organs may be developed at 

 points which in other forms only carry sensory organs of a lower kind." 



