b EXTERNAL FEATURES. 



Viewed externally, the prosobranchiates, as well as the mol- 

 lusca 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. Tims we find that at first the 

 Minis is at the posterior end of the body, but gradually it ap- 

 proaches nearer MIX I nearer to the anterior end until finally it 

 opens on the back or right side near the mouth. In its early 

 stages the shell and mantle only occupy the extreme posterior 

 end of the body, but in their growth they cover more nnd more 

 of the latter, gradually pushing- the anus forward. 



An external shell, usually sufficiently large to contain the 

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

 secretion of the mantle, and conforms to its shape; and the hard- 

 ness which it assumes by the addition of carbonate of lime. 

 renders it an etlicienl protection to the animal, whilst the faith- 

 fulness with which il reproduces the external features 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 possible 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 portion of the body, the posterior dorsal 

 part of the foot bears an operculum, secreted by an expansion of 

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

 a spiral also, but in the same plane ; yet in many cases its growth 

 is annular. Usually horny, 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 completely closes. Like 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- 



