THE ORGANS. 45 



layer of rods is convex there are, as a rule, as many lenses as there 

 are perceptive nerve-endings ; when the layer of rods is concave, 

 there is one lens only. By the addition of other arrangements to 

 the nervous apparatus of the eye, by which its functional capacity 

 is modified or increased, this organ becomes one of the most com- 

 plicated of the animal economy. In most of the lower divisions the 

 optic organ, even when fully developed, still retains its primitive 

 relation to the ectoderm. In the higher divisions it is separated 

 from it, and gets to lie, together with its perceptive apparatus, be- 

 neath the integument, or the perceptive apparatus is derived from 

 the embryonic foundations of the nerve-centre. 



The phenomena of differentiation may be seen even in what 

 relates to the position of the optic organ, for the parts of the body 

 which carry the eyes, as well as the number of the eyes, varies 

 greatly in the lower divisions of Animals. Connected with this is the 

 occurrence of a great number of eyes on the anterior part of the 

 body, which goes to form the head, until, finally, the number of 

 eyes on the part in question becomes limited to two. The different 

 position occupied by visual organs forbids us to suppose that they 

 have had a common hereditary origin, and is in favour of these 

 heterotopic organs having been independently differentiated from 

 an indifferent apparatus. On the other hand, that eyes which are 

 connected to the cerebral ganglion, or the dorsal nerve-centre, have 

 a common genetic relation, is not to be disputed. 



f) Respiratory Organs of the Integument. 

 (Dermal Branchise.) 



§ 39. 



An important part is played by the integument, and therefore by 

 the ectoderm, in the formation of respiratory organs. Before 

 they appear the gas exchange is carried on probably by the whole 

 surface of the body, and this mode of respiration obtains in many 

 of the lower aquatic animals. A change of the surrounding medium 

 is effected, partly by the movements of the body, and partly by 

 special organs, for example cilia ; thus fresh quantities of it are con- 

 tinually brought into contact with the respiratory surface. This is 

 not, however, the only method of respiration in the lower animals, for 

 the introduction of water into the interior of the body, in fact 

 the bathing of the alimentary canal by watei', is certainly not 

 without significance in this direction, while it is of great im- 

 portance as being the beginning of a long series of differentiations. 

 Certain limited portions of the surface become more developed in this 

 direction as the function becomes localised, and, in compensation 

 for this limitation, acquire the form of blood-carrying processes, 

 which are called branchiae. In many cases they are differentiated 



