148 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



possible description, perhaps not more than a third of a millimetre- 

 in depth. The surface of the shell itself is crowded with 

 perforations, giving it the cribriform appearance which is familiar 

 to us in the case of the Foraminifera. Small as these perforations 

 are, there seem to be still finer pores between them. The nerve- 

 fibre can often be discerned as it were blocking up the tube. One 

 can easily understand how it is that nacreous shells so easily perish 

 and fall away into powder : they are so perforated and tunnelled 

 that all idea of their solidity is lost, and one wonders how they 

 hold together as they do. 



In the case of the mantle-eyes and the dark pigmented matter 

 the evidence is not always so clear. Though a study of the eyes in 

 the mantle of species Pecten and Area has shown us beyond doubt 

 that there are optical organs of this kind ; yet somewhat similar 

 structures have been found in other species which can only be 

 doubtfully regarded as organs of vision. Thus, for instance, in a 

 European species of oyster, certain pigment balls have been observed 

 beneath the epidermis with tentacles which are nearly pigmentless 

 and ciliated. There is also pigment scattered irregularly over the 

 surface, which in some way subserves the purposes of vision. 

 These cells or similar ones have been found in the European 

 Patella. Two kinds of cell have been distinguished ; one of which 

 is supposed to be glandular and to secrete the cuticula, while the 

 other is sensitive to light. Similar organs have been found on 

 Mediterranean species of Pinna and Avicula, which Will described 

 as eyes, and Carriere regarded as glands. The able American 

 anatomist W. Patten, however, found that the organs consisted 

 of an immense number of conical cells expanded at the outer 

 extremity and drawn out to a point at the inner. They were 

 filled with a mass of refracting, closely-packed cells, which gave 

 them the appearance of gland-cells, and were surrounded by narrow 

 ciliated cells, sometimes faintly coloured at their outer ends. 

 Without going into technical detail, which would be difficult to 

 understand without an explanation of terms, the following may 

 be regarded as Mr. Patten's conclusions. The position of these 

 organs on the edge of the mantle and their hemispherical shape 

 resemble closely the facetted eyes of Area; but they have no 

 pigment and the nervation is unknown. But experiments made 

 on Pinna and other Mollusks, show that they possess sensitive 

 powers of vision. In the case of Avicula the least shadow causes 

 it to close its shell quickly with a force that indicates considerable 

 nervous disturbance. 



With reference to these novel and anomalous sense-organs we 

 must not expect that a uniform type will be followed, and this has 

 been so early recognized by observers that a new series of terms- 

 has been devised. Instead of distinguishing certain structures by 



