GENERAL CHARACTERS OF MOLLUSCA. 327 



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 mass of Coral ; and yet these exhibit a 

 complex and powerful digestive apparatus, a regular circulation 

 of blood, and an active respiration. But we nowhere find, 

 throughout the whole animal kingdom, that the conformation of 

 these organs governs the shape of the body ; they rather adapt 

 themselves to the type which predominates in its structure, and 

 which is principally manifested in the disposition of the loco- 

 motive organs. Thus, the stomach of the Star-fish sends a 

 prolongation into each ray; whilst in the Articulata, on the 

 other hand, we find the digestive cavity prolonged into a tube, 

 in accordance with the form which the body there possesses. 



936. Thus we see that, in regard to external shape and 

 arrangement, the apparatus of Organic life has no definite plan 

 of its own ; and in the Mollusca there is an absence of any 

 general type, to which it may be made conformable. Hence the 

 shape of the body varies extremely in those classes, in which it 

 is entirely or principally composed of these organs; and no 

 general character can be given, which shall apply to all, or even 

 a large part, of the animals composing them. There is often an 

 entire want of every kind of symmetry ; that equality of the two 

 sides, which is peculiarly characteristic of the higher animal^ 

 being deficient ; as well as the radial arrangement of parts seen 

 in the lower. But this is only the case, where there is no 

 development of a head ; that is, of a prominent part on which 

 the mouth is situated, and which also bears the organs of sensa- 

 tion, if any exist. In the higher Mollusca, which possess not 

 only sensory tentacula, but eyes, and even organs of smell and 

 hearing, we find these disposed in a symmetrical manner ; so 

 that the head (where it exists), or the part peculiarly concerned 

 in animal life, presents a bi-lateral equality of parts, even where 

 the remainder of the body wants it. In the more active species 

 among the higher classes, we find this bi-lateral symmetry 

 extending in many instances through the whole body ; evidently 



