114 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



external differences except those scarcely specific in character, not 

 only distinct genera, but even three distinct families. In this 

 there is nothing very surprising, if we remember that the early 

 condition of embryonic shells is cup-shaped. This persists in some 

 by their growing regularly, and thus they have in the adult 

 condition a more or less elevated and conical shape. The difference 

 between these forms and the whorled, spiral, or heliciform shells, 

 is derived from the fact that the embryonic form developes 

 disproportionately in one direction. The mode of this development 

 gives rise to all those modifications of form which are met with 

 in the Mollusca. From this it may be remarked in passing, how 

 useless any system of classification must be which confines itself 

 exclusively to the form and color of the shell. Thus restricted, 

 conchology was for a long time unworthy of the name of a science. 



But while we recognize the great anatomical differences which 

 separate animals which resemble one another in the forms of their 

 shells, we must note the fact that similarity of habits or the 

 conditions of life lead to other resemblances of an important 

 kind. This will be seen from the observations I shall have to 

 make on the eyes I have found in the shells inhabiting the littoral 

 regions. These seem to be in number, size, and disposition, very 

 similar in all the littoral shells that congregate about the tidal 

 marks. I have found that these organs of vision are reproduced 

 in the same manner, or to some extent the same manner, when 

 the shell is subject to much erosion from the alternate action of the 

 air, sun, and salt water. I have found this the case particularly 

 in the genera Patella, Acmcea, and Siphonaria, all conically 

 shelled species : as well as the spiral univalves Trochus, 

 Trochocochlea, Senectus, Littorina, and Risella. These instances 

 will be referred to in detail further on. 



Before proceeding with this matter of the shell, it is advisable 

 to deal with some points of classification, which illustrate in the 

 conically shelled species important physiological and anatomical 

 principles. We find amongst the conical-shelled littoral species, 

 three or perhaps four forms of branchiae or respiratory organs. In 

 Patella they are a fringe round the foot, between it and the 

 mantle. It is interrupted only for a short distance where the 

 head protrudes. In Acmcea the gill forms a single pectinated 

 plume at the back of the neck. In Siphonaria there is a lateral 

 respiratory orifice leading to a chamber which is covered by a 

 portion of the mantle, forming a pulmonary cavity like that which 

 obtains amongst the slugs and snails, or rather like those Mollusks 

 which are destined to breathe both air and water. In the case of 

 Siphonaria there is a gill in the pulmonary cavity. 



The function of respiration is generally speaking a function of 

 the integument, and is said to be a gill or lung when it is specially 



