V. CEPHALOPODA. 



385 



equal in size (fig. 381), four on the right side, four on the left. 

 The Decapoda (fig. 382) have in addition two longer arms which 

 bear suckers only on the enlarged tips and can be retracted into 

 special pouches. This additional pair come between the third 

 and fourth of the Octopoda, counting from the dorsal side. 



Behind the crown of tentacles are, right and left, the pair of 

 large eyes which superficially closely resemble those of the verte- 

 brates, since they have a transparent cornea and a large pupil sur- 

 rounded by an iris. Internally the resemblance is not less pro- 

 nounced (fig. 383). Behind the iris is a lens and a vitreous body, 



A Int 



FIG. 383. 



FIG. 384. 



FIG. 383. Diagrammatic secticm of Cephalopod eye. (After Gegenbaur.) ae, argentea 



(choroid) ; C, cornea ; ci, ciliary process ; go, optic ganglion ; ik, iris ; fc, cartilages; 



_L, lens ; p, pigment layer ; Re, cellular layer of retina ; JJr, rod layer of retina; 



w, white body. 

 FIG. 384. Schematic section of eye of Nautilus. (From Balfour.) A, aperture of 



optic cup ; Int, iris-like fold of integument ; JV.op, optic nerve; .R, retina. 



the latter being bounded by the retina and this in turn by a pig- 

 mented silvery layer, the argentea or choroid, which contains 

 cartilages recalling the sclerotic coat. Two striking peculiarities 

 separate these eyes from those of the vertebrates and show that 

 they have arisen independently and have an entirely different de- 

 velopmental history. (1) The cornea in many species has an 

 opening by which water enters the anterior chamber; (2) the layer 

 of rods in the retina abuts against the vitreous body and the gan- 

 gl ionic layer lies behind, while in the vertebrates the reverse is 

 the case. 



