EXTERNAL FEATURES. 9 



1. Head. This is sometimes partially separated from the body- 

 by an intervening constriction or neck, but is very frequently 

 only indicated by the possession of a mouth on its ventral and 

 of a pair of tentacles on its dorsal surface. From the latter 

 sometimes proceed pedicels bearing the ej^es. 



2. Mantle. This covers the posterior portion of the dorsal 

 surface of the body. 



3. Foot. The ventral surface of the body, or a specialized por- 

 tion of it. 



The mouth may be either simple, in the plane of the head, or 

 it may be produced at the end of a contractile rostrum, or of a 

 retractile proboscis (xvi, 91). Behind the head and on the under 

 side of what may be called the neck is attached the foot, and 

 behind the neck begins the posterior portion of the body covered 

 by the mantle ; this part of the animal is very long, usually, and 

 as the shell is moulded upon it, it assumes a similar form — 

 generally spiral. However, in Patella and similar limpet-like 

 shells, the body is short and straight. The mantle encircles the 

 body like a collar below, but above it is much produced over 

 the back of the animal, and the respiratory cavity forms a sinus 

 of its margin. The foot consists of the attachment to the neck, 

 which is usually narrow, and an expanded portion, sole, or 

 creeping disk. Such is its typical form, but in Patella, etc., the 

 foot includes the entire ventral surface of the animal. It is a 

 muscular process of the body, and the only locomotive organ 

 possessed by gastropods. 



Viewed externally, gastropods, as well as the mollusca 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 their 

 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 the body, but gradually it 

 approaches nearer and 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 and more 

 of the latter, gradually pushing the anus forward. 



An external shell, usually sufficiently large to contain the 

 entire animal, is common to most Gastropoda. It is a secretion 

 of the mantle, and conforms to its shape ; and the hardness 

 which it assumes by the addition of carbonate of lime, renders 

 it an efficient protection to the animal, whilst the faithfulness 

 with which it reproduces the external features of the latter 

 renders it extremely useful in classif3dng the mollusca. The 

 spiral growth of shells is as nearly of true mathematical 

 regularity as is possible in an organic body ; forming the loga- 



