l> EXTERNAL FEATURES. 



Viewed external!} , the prosobranchiates, as well as the mol- 

 lusea in general, must be regarded as bilaterally symmetrical ; a 

 view which is strengthened by the position of the nervous 

 system ; and the developmental history of the intestines, though 

 its actual position, as well as that of the sexual organs, does not 

 correspond with this symmetry. Thus we find that at first the 

 anus is at the posterior end of tlie body, lint gradually it ap- 

 proaches nearer and nearer to the anterior end until finall}^ it 

 opens on the back or right side near the mouth. In its earl}' 

 stages the shell and mantle only occupy the extreme posterior 

 end of the body, but in their groAvth they cover more and more 

 of the latter, gradually pushing the anus forward. 



An external shell, usually sutticiently large to contain the 

 entire animal, is common to all the prosobranchiates. It is a 

 secretion of the mantle, and conforms to its sliape; and the luird- 

 ness which it assumes l»y the addition of (carbonate of lime, 

 renders it an efficient protection to the animal, wliilst the faith- 

 fulness with which it reproduces the external fentures of the 

 latter renders it extremely useful in classifying the mollusca. 

 The spiral growth of shells is as nearly of true mathematical 

 regularity as is i)Ossible in an organic body, forming the loga- 

 rithmic spirals of Moseley. or conchospiral of Naumann. Cor- 

 responding to the shell, which is produced by the mantle, and 

 borne by the posterior i)orti(m of the body, the posterior dorsal 

 part of the foot bears an operculum, secreted by an exi^ansion of 

 its skin called the opercular mantle. Typically, the operculum is 

 a spiral also, but in the same i)lane ; yet in many cases its growth 

 is annular. I^sually liorny. it is sometimes nearly, entirely or 

 partially calcareous, and on the retirement of the animal within 

 the shell it is brought into the aperture of the latter, which it 

 more or less completelj'^ closes. Ivike the shell itself, it may be 

 considered a protective organ, and when in apposition with the 

 former suggests the two enclosing valves of the lamellibranchiate 

 or bivalve mollusks. 



Notwithstanding the large portion of the animal which is 

 always within the shell, even when the head and foot are ex- 

 truded, the latter is only attached to the former at one point, on 

 the columella, and by means of a columellar muscle, which, pass- 

 ing through the foot, is attached at its other end to the oper- 



