72 MOLLUSCOUS TYPE OF STRUCTURE. 



go from place to place in search of food, instead of waiting for 

 what is brought to them by the currents, which the action of their 

 cilia creates in the surrounding water. Now, in these animals 

 we always find the head possessing a symmetrical arrangement of 

 its parts, the eyes, tentacula (or feelers), &c., being arranged in 

 pairs, on the two sides of the central line, as seen in Fig. 31. 

 But the remainder of the body, which contains the organs of 

 nutrition or vegetative life, is very commonly unsymmetrical 

 externally, being disposed in a spiral which fills the interior of 

 the shell, and having a complete want of uniformity in the 

 arrangement of the organs themselves. 



49. In the species which are destitute of shell, however, or 

 which have but a small one that only partially covers the body, 

 we find the symmetry more complete, and 

 the powers of motion greater. This is 

 the case, for example, with the group of 

 Pteropods, of which an illustration is given 

 in Fig. 32. Here we see that the two 

 sides of the body ure exactly alike exter- 

 nally ; and the correspondence extends in 

 great degree to the internal organs also. 

 The movements of these animals through 

 the water are comparatively active ; and 

 they remind us strongly of those of Insects 

 or Birds, to which they may be considered 



as having a decided analogy (4). Still, the general rule 

 holds good, in regard to them also, that their muscular system is 

 but little developed; the several bands and fibres of which it 

 consists, having no firm points of attachment ; and bearing no 

 proportion, either in number, or in the variety of their actions, 

 to those of even the lowest Articulata. This is the case also 

 with the Cuttle-fish tribe, which presents many characters that 

 lead us towards the Vertebrated series, and which depart widely 

 from those of the typical Mollusks, the essential peculiarity of 

 whose organisation, is unquestionably the low development of the 

 organs of animal life, in comparison with those of nutritive or 

 v -'jetative life. 



