24 DIGESTIVE ORGANS. 



enormous liver, filling nearly the whole of the first whorl of the 

 shell, and pouring its secretions into the former (and often into 

 the latter also) by several openings. We will successively ex- 

 amine these various organs more in detail. 



The proboscis is a production of the skin of the anterior or 

 head portion of the body, bearing the mouth at its end. When 

 it remains permanently protracted it receives the name of ros- 

 trum, that of proboscis being more properly limited to this organ 

 when provided with muscles by which it can be retracted within 

 the body. The typical proboscis is quite characteristic of the 

 siphonostomated Prosobranchiates, or those carnivorous mol- 

 lusks of which the shell is canaliculate or notched at its lower 

 extremity; whilst those animals provided with a rostrum or snout, 

 or with a simple mouth are members of the usually phytophagous 

 holostomata In Dolium, a remarkable exception, the exceed- 

 ingly long proboscis accompanies phytophagous habits. 



The invagination of the proboscis is effected by means of pow- 

 erful retractor muscles supplied along its entire length and espe- 

 cially numerous at its base, where the retraction begins. The 

 protrusion of the proboscis, on the contrary, is effected by press- 

 ing forward the blood towards the head, an operation assisted by 

 the contraction of the annular muscles of the fore-part of the 

 body. In Natica, according to Troschel, the invagination of the 



naming and technical description of species, in the same manner dis- 

 tinguishes for us the implements which we should use in our investigation 

 of nature implements by which the Great Builder has worked, in which 

 He has expressed His thought. The proper acquisition of a language 

 requires the preliminary knowledge of its grammar, the knowledge of 

 letters precedes reading : even so, the simple name of a species, then of a 

 genus, and its recognition when met with or referred to forms the mere 

 alphabet of science, from which we proceed gradually to the consideration 

 of individual properties, then to intimate and to wider relationships, 

 until we are fortified with sufficient knowledge to generalize. In these 

 latter days generalizations are numerous enough also, but unfortunately 

 they are usually the product of minds not furnished with the requisite 

 intimate knowledge of the factors with which they build their theories. 

 The details of anatomy and physiology of the mollusca will be given for 

 each group as it occurs in the course of this work, and I shall thus neces- 

 sarily repeat myself frequently; and I hope that by the time that my 

 reader and I have finished the volumes containing the technical descrip- 

 tions, we will both be better prepared than we are at present to under- 

 stand the subject as a whole. This course has also the important personal 

 advantage, that if at any time I shall be attacked for sins of commission 

 or omission, I may make the obvious defense wait and see. 



