PROSOBRANCHIATA. 



ANATOMY. 



External Featureii. 



The body iu prosobranchiates presents the followino-, more or 

 less, diftereiitiated portions : 



1. Head. This is sometimes partially separated from tlie 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 eyes. 



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

 surface of the body. 



3. Foot. Arising from the ventral anterior ])art of the liody. 



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

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

 retractile proboscis. Behind the head and on the under side of 

 what ma}' 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 i)roduced 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 or sole, or creeping 

 disk. Such is its tj-^pical form, but in Patella the foot embraces 

 the entire ventral surface of the animal. It is a muscular pro- 

 cess of the bod}^, and the only locomotive organ possessed hy 

 prosobranchiates. 



