SECT. VI THE SENSORY ORGANS 109 



admit the relationship between Limulus and Apus, 

 and deduce them both from the same bent AnneHd, in 

 which the eyes wander from the prostomium, where 

 they are useless as eyes, to a position where they can 

 function as such. In Part II. we hope to show that 

 the derivation of Apus and that of Limulus from the 

 same bent Annelid stand or fall together. 



Function. — As to the function of the unpaired eye, 

 we can perhaps with some certainty conclude that in 

 Apus at least it regulates the position of the body in 

 the water. Its structure out of four sensory retinae at 

 once suggests such a function, while further, the loose 

 tangle of pigment cells would constitute a body free to 

 be acted on by the earth's attraction. The organ is 

 perhaps rendered more perfect by the rounded arrange- 

 ment of the retinal cells, which thus present many 

 different surfaces to appreciate the movement of the 

 mass of pigment cells under the action of gravity. 



It is difficult to ascertain for certain whether the 

 chitinous canal opens in the cavity of this sensory 

 organ or not. If it opens in the cavity to fill it with 

 water we should rather expect to find a more definite 

 membrane round the whole organ. If, on the other 

 hand, it does not open, it may be a structure for 

 the appreciation of changes of pressure, i.e., of 

 depth. The end of the tube as shown in Fig. 27 

 (2 and 3) is irregular, and in section seems as if it 

 might be a loose empty sac. As the outer pressure 

 increases, such a fine membranous sac would be the 

 first part of the body to feel it, and would commence to 

 swell. But such an appreciation of changes of pressure 



