56 COMPARATIVE ANATOMY chap. 



joints, overlapping in such a way as to be movable one upon the other. 

 This segmented shell can protect the whole body, since it allows the 

 Chiton to roll up like an Armadillo or a Wood-louse. 



In the Lamellibranchia, the protection of the whole of the soft body 

 is provided for by the development of a bivalve shell, from which the 

 foot can be protruded, and which, by the closing of its two valves, 

 completely envelops the soft body as well as the retracted foot. 



In the Gastropoda, Saiphopoda, and Cephalopioda, the most complete 

 protection on all sides of the body by means of the shell is attained 

 on another plan. The shell becomes much elongated and turret- 

 like, and is thus so capacious that not only the visceral dome but the 

 head and foot also can find place in it. Even the only remaining 

 unprotected aperture, the one weak point of this fortification, can very 

 often be completely closed by a hard operculum. 



A long, turret-like shell is an inconvenient burden for a freely 

 moving animal, being, in consequence of its large surface, a hindrance 

 to locomotion. A reduction of the surface is brought about in the 

 Gastropoda and Cephalopoda by the coiling of the shell, either in one 

 plane or in a conical spiral. 



In the latter case the spiral twist is almost always right-handed or 

 dextral. 



Ill order to decide the direction of the twist, the shell should be held in such a 

 manner that the point of the spiral is uppermost, while the aperture is directed 

 downwards and towards the observer (Fig. 60, p. 60). If, in this position, the 

 aperture lies on the riglit of the axis, tlie shell has a dextral twist if to the left its 

 twist is left-handed or sinistral. 



We have a striking and in most cases unexplained phenomenon in 

 the reduction and even complete disappearance of the shell, which takes 

 place not only in nearly all the classes, but even within some of the 

 smaller groups of Molluscs, e.g. the Solenogasires among the Amphineura, 

 a few Heteropoda and Titiscania among the Prosohranchia, many Pid- 

 mowda, very many Opisthohranchia, and most extant Cephalopoda. 



In almost all cases the forms in which the shell is rudimentary or 

 wanting can be shown to be derived from forms in which it is well 

 developed. All shell-less snails (slugs) have shells in the early stages 

 of their development. 



The process of the gradual reduction of the shell to a rudiment, 

 which will be more fully described later on, is often as follows : (1) 

 the shell becomes internal ; (2) it decreases in size, so that it no 

 longer can cover the body ; (3) the visceral dome disappears ; (4) the 

 shell is only to be found in the form of isolated calcareous particles 

 in the dorsal integument ; (5) even these vanish, and the shell is only 

 to be found in the embryo. 



Only in a few cases is it possible distinctly to recognise the reason 

 or the advantage of this reduction of a protective covering so useful 

 to and exercising so profound an influence on the organisation of the 



