GENERAL CHARACTERS OF MOLLUSCA. 45 



nished with tentacula for drawing food to its orifice.* The nutrient materials 

 are imbibed by the walls of the stomach, and are transmitted by them to the 

 tentacula, without any regular circulation ; and the exposure of the whole of 

 the soft surface of the body to the surrounding liquid, affords all the aeration 

 which is requisite. In the Medusae, or Jelly-fish, we often find the stomach 

 extending itself into a ramified system of tubes, which convey its contents to 

 the thin border of the umbrella-shaped disk, for more effectual aeration ; but 

 there is still no separate circulating system, except in a few instances. In the 

 class of Echinodermata, however, which includes the highest forms of Ra- 

 diated animals (such as the Asterias or Star-fish, Echinus or Sea-Urchin, and 

 Holothuria or Sea-Cucumber), we find the digestive cavity restricted within 

 much narrower limits ; and there is here a distinct system of vessels, adapted 

 to absorb the nutrient fluid from the digestive cavity, and to convey it to the 

 remoter parts of the system for their nutrition, as well as to effect its aeration, 

 by exposing it to the influence of the air contained in the surrounding liquid, 

 in organs especially adapted for that purpose. 



4. General characters of Mollusca. 



11. The range of Animal forms comprehended in the Sub-Kingdom MOL- 

 LUSCA is so great, that it would be difficult to include them in any positive 

 definition which should be applicable to all. They present few traces of the 

 circular disposition of organs around the mouth, which is characteristic of the 

 Radiated classes ; and we seldom meet with any marked approach to the 

 elongation of the body, still seldomer with any indication of that division 

 into segments, which are the chief peculiarities of the Articulata. It is by 

 the absence of these, and of any trace of the Vertebrated structure, that the 

 Mollusca are most readily defined. The variety of form which they present, 

 is less surprising, when it is considered that the bulk of their bodies is almost 

 entirely made up by organs of nutrition ; the organs of sensation and locomo- 

 tion, which they possess, being chiefly subservient to the supply of these. 

 We find, in the lowest tribes of this group, living beings which are fixed to 

 one spot during all but the earliest period of their lives ; and which scarcely 

 possess within themselves so much power of movement, as that enjoyed by 

 the individual Polypes in a compound polypidom ; and yet these exhibit a 

 complex and powerful digestive apparatus, a regular circulation of blood, and 

 an active respiration. We never find, throughout the whole Animal kingdom, 

 that the apparatus of organic life is arranged on any definite plan of its own ; 

 its confirmation being adapted to the type which predominates in the struc- 

 ture of each group, and which is principally manifested in the disposition of 

 the locomotive organs. Thus, the stomach of the Star-fish is circular, and 

 sends a prolongation into each ray ; whilst the digestive cavity of the Articu- 

 lata is prolonged into a tube. In the Mollusca, there is no such definite 

 type, the apparatus of nutrition having the predominance over that of loco- 

 motion ; and the form of the body is, therefore, extremely variable. The re- 

 lative places, even of the most important organs (such as the gills), are found 

 to undergo complete changes, as we pass from one tribe to another ; although 

 their general structure is but little altered. 



12. The lower Mollusca may be characterised as consisting merely of a 



* It is usual to speak of the orifice of the stomach, in the Polypes, as the mouth ; and to 

 regard the tentacula as prolonged lips. It appears to the author much more reasonable, how- 

 ever, to consider this aperture as the cardiac orifice of the stomach ; and to regard the tenta- 

 cula in the light of pharyngeal constrictors, their office being to grasp the food and convey it 

 to the stomach. This view is borne out by the conformation of the superaddcd parts in the 

 Ciliobrachiate Polypes and Ascidian Molluscs. 



