THE MOLLUSC A 



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. Tin's 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 Digestive 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 Entosiphon ; the alimentary tract is rudimentary in 

 the parasites Entocolax and Entoconcha ; it is absent altogether in 

 Enteroxenos. 



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 chitirious, and are only calcified in certain Chaeto- 

 dermidae, and partially so in Nautilus. 



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, 



