vii MOLLUSCA THE SHELL 55 



1 . Apparatus for locking the mantle. These are paired or unpaired. The former 

 are to be found on the posterior side of the body, in the mantle cavity, near its 

 lower end ; they lie to the right and left at the base of the funnel, and on the 

 corresponding points of the inner surface of the mantle fold. The unpaired, on 

 the contrary, are found on the anterior surface of the neck. Since all the arrange- 

 ments serve the purpose of cutting off the mantle cavity from the external medium, 

 it is easy to see that their development is in inverse ratio to the extent of the con- 

 crescence of the edge of the mantle round the neck before mentioned. Where no 

 concrescence is found, as in Sepia, the arrangements for locking the mantle are 

 highly developed ; while, where the line of concrescence is very long, as in Octopus, 

 the locking apparatus may be altogether wanting. The locking apparatus consists, 

 in general, of cartilaginous prominences (often accompanied by depressions) on the 

 inner surface of the mantle fold. i.e. the surface turned towards the mantle cavity, 

 which exactly fit corresponding cartilaginous depressions accompanied, as the case 

 may be, by prominences, on the opposite body wall (cf. Fig. 80). The special forms 

 of the mantle and nuchal locking cartilages are of importance in classification. 



The cartilaginous arrangements for locking, which are almost always found in the 

 Decapoda (they are wanting only in Oivenia and Oranchia), are still retained in a 

 few Octopoda in the form of more or less modified fleshy processes (Argonauta, 

 Tremoctopus}. The nuchal locking apparatus is the first to disappear on the rise of 

 the pallio-nuchal concrescence. It has disappeared among the Decapoda in the 

 genus Scpiola, where the mantle is firmly attached to the neck. 



2. Permanent connections between the mantle fold and the adjacent body wall 

 traversing the mantle cavity are found only in those Cephalopods which have no 

 locking apparatus. Thus in Octopus and Eledonc the mantle is attached to the body 

 wall by means of a median muscle above the funnel. This muscle consists of two 

 closely - applied lamellae, having the anus between them. In Cranchia the free 

 dorsal edge of the funnel (at its so-called base) has become united by an integu- 

 mental band on the right and left with the mantle fold, and a similar arrangement 

 is found in Loligopsis. 



Water pores. Near the mouth, or at the bases of the arms, or laterally on the 

 head, in many Cephalopods, there are apertures leading to integumental pouches of 

 varying size. The function of these organs is unknown. 



IV. The Shell. 



General. 



The Shape of the Shell, and its Relation to the Soft Body. 



All the various forms of shell found in the Mollusca are deducible 

 from a cup- or plate -like shell covering the dorsal region. Such a 

 shell affords sufficient protection for animals such as Fissurella, Patella, 

 etc., which can firmly and almost immovably attach themselves to a 

 hard surface by the sucker-like foot. The soft body is in this case 

 protected on one side by the shell, and on the other by the surface 

 of attachment. Free -moving Mollusca, however, show a tendency 

 to protect the whole body exclusively by means of their shells, and 

 this object is attained in various ways. 



In the Chitonidce, for instance, the shell is made up of consecutive 



