ORGANS OF THE SENSES. 253 



part of the body, and the same arrangement is seen in the 

 planaria nigra, which corresponds with the circular arrange- 

 ment of the same organs around the margin of the mantle 

 in some of the most active conchifera. The same transverse 

 arrangement is seen in the ten prominent eyes disposed 

 across the head of the medicinal leech. In the bdella nilotica 

 there are six eyes disposed transversely on the upper part of 

 the first segment, in the polynoe impatiens they are disposed 

 laterally in pairs. The optic nerves of some of the higher 

 annelides are observed to terminate in a broad circular pa- 

 pilla or retinal expansion. The choroid pigment behind the 

 crystalline lens in the medicinal leech is red in the young 

 animal and changes to black in the adult state, as observed 

 in many other animals. In the nereis nuntia (Fig. 14) there 

 are four large eyes disposed symmetrically in two pairs on 

 the upper part of the head, and nearly a hundred smaller 

 ocular points disposed in rows and groups on all the promi- 

 nent lobes around the mouth. In the syllis monilaris, there 

 are two pairs of eyes with prominent glistening cornese dis- 

 posed on the upper segment of the head. In some of the 

 higher forms of this class, however, as the euphrosine laureata 

 and the oenone lucida, the visual organs are reduced to two, 

 symmetrically disposed on the upper part of the first seg- 

 ment of the head, and thus approach to the normal character 

 of these organs in most of the higher classes of animals. 



In the entomoid classes of articulata, the most active of all 

 the invertebrated animals, the organs of vision are very nume- 

 rous and commonly aggregated together to form two groups, or 

 two compound eyes symmetrically disposed on the upper or la- 

 teral parts of the head a compound character which com- 

 mences as low as the rotifera by the approximation of the se- 

 parate ocular points. The eyes of the myriapods partake some- 

 times of the character of those of the inferior annelides and 

 sometimes of those of the higher forms of insects, and in 

 some species no organs of vision have been detected. Most 

 of these animals present numerous separate simple eyes 

 grouped together on the two sides of the head. The two 

 lateral eyes of the scolopendra consist each of a group of 

 about twenty-three small distinct eyes approximated and 

 placed in lineal rows, and the aggregate eyes of the iulus are 

 also composed of several rectilineal rows. The scuttgera has 



