6 THE MOLLUSCA 
thickness. Certain portions of the tegumentary organs can be 
spontaneously detached from the body by the reaction of the 
animal against the incidence of external forces. This phenomenon 
is known as “autotomy,” and is generally defensive, as, for instance, 
~ the loss of portions of the foot (Harpa) or of its appendages ; the 
loss of the siphons of some Lamellibranchs ; of the dorsal papillae 
and other parts of the dorsal integument in certain Nudibranchs ; 
of the cephalic tentacles of Scaphopods, etc. The lost parts are 
regenerated, just as other tegumentary organs (cephalic tentacles, 
fins, arms of Cephalopods) that have been accidentally removed, are 
regenerated with all the complex and differentiated structures that 
they may possess, such as eyes, suckers, etc. The most remarkable 
example of regular physiological autotomy is found among the 
Cephalopods, namely, the hectocotylised arms of the Philonexidae 
and Argonautidae (Fig. 287). 
2. The Digestiwe Tract—The alimentary tract always has two 
orifices, the mouth and anus, generally situated at the two 
extremities of the body; but the anus may be brought to an 
anterior position by a ventral flexure, which may or may not be 
complicated by a lateral torsion. The anus is absent only in the 
parasitic genus HLntosiphon ; the alimentary tract is rudimentary in 
the parasites Hntocolax and Hntoconcha; it is absent altogether in 
Einteroxenos. 
In all other forms three essential parts can be recognised in the 
digestive apparatus. Firstly, the buccal or anterior section of the 
gut, of ectodermic origin, which comprises the first dilatation or 
buccal cavity and the oesophagus. Secondly, there is the mid-gut, 
of endodermic origin, comprising the second dilatation or stomach. 
Thirdly, there is the hind-gut or intestine. The anterior dilatation 
or buccal cavity is absent in the Lamellibranchs, with the exception 
of certain Nuculidae. Cuticular formations are present in different 
parts of the internal wall of the alimentary tract, and are specially 
developed in its anterior portion. They occur around the mouth— 
having the form of a prehensile collar in Doris—but more 
particularly in the buccal cavity, where two different sorts of 
cuticular formations are found, the mandibles and the radula. The 
mandibles are anterior, dorsal, and unpaired in the Dentaliidae, 
Patellidae, Pulmonates, Aegirus (Fig. 73, B), etc, but paired and 
lateral in the majority of the Gastropoda. In the Cephalopoda the 
members of the pair are dorsal and ventral (Fig. 266). These 
organs are chitinous, and are only calcified in certain Chaeto- 
‘dermidae, and partially so in Nautilus. 
) ] Me 
The radula is characteristic of the phylum Mollusca. It exists 
throughout the series, from the most archaic forms upward, and is 
only absent in the most specialised types, in which it has evidently 
been lost, such as certain Neomeniomorpha among the Aplacophora, 
