ANNULATA. 251 



objects, of making towards some point, and of avoiding others. In 

 the absence of such an optical arrangement as may be fitted to collect 

 the rays of light, the physiologist, however, can form no conception 

 of the mechanism of sight in these animals, if this endowment really 

 is conferred upon them, under the conditions and in conformity with 

 the laws which atfect these organs in all the higher animals. It were, 

 however, by no means irreconcileable with the views entertained at 

 present as probable by many philosophers with reference to the pro- 

 perties of light, to suppose that this subtile agent may be rendered 

 perceptible to the sensorium of the humblest animals, by means of a 

 mechanism very different from that which anatomists and opticians 

 recognise in the apparatus of an eye. It is not essential to the practical 

 purposes of the lowest forms of life, that the objects of the external 

 world should be seen, that pictures of them should be painted upon 

 the retina ; it were enough that the mere presence or absence of an 

 objective body should become evident to the sensations of the animal 

 by the positiveness or ncgativeness of the impressions received. A 

 refinedly exalted sense of touch, tactile sensibility, would suffice to 

 accomplish this object. It is not easy for those who have never enjoyed 

 the spectacle of the ' feat of touch,' performed by the tentaculated 

 worms, to estimate adequately the extreme acuteness of the sensi- 

 bility which resides at the extremities of the living threads with 

 which the head and sides of the body are garnished. They select, 

 reject, move towards and recede from minute external objects with 

 all the precision of microscopic animals gifted with the surest eagle- 

 sight. The necessity, therefore, of ordinary organs of sight, in the 

 present state of physical and physiological science, it is by no means 

 essential to admit, while acknowledging that under the agency and 

 stimulus of light the humblest beings, though unendowed with normal 

 visual organs, may yet steer themselves harmlessly, readily, and un- 

 erringly through the thickly tangled labyrinth of mud and stone, and 

 gravel and weed, amid the twilight of which the habitat of many of 

 them may have been cast. 



"It is a remarkable fact in the history of the AneUids, that scarcely 

 any species, however organised, whether furnished or not with exter- 

 nal locomotive organs, in its numerous and varied muscular evolutions 

 ever moves directly backivards. The movements consist always of 

 serpentiform pranks, or those of elaborate coiling. It is by the head, 

 however, that the movement is invariably initiated. The tail is 

 always in the rear, never in the van." * 



On the first appearance of the embryo anellid, it usually consists of a 

 single segment, which is chiefly occupied by a large mass of unmeta- 



* CXC. p. 268. 



