EVIDENCES— EXISTING ORGANISMS 



113 



dermal layer. In the former the lens is derived independently 

 from the outer ectoderm, but in the latter it comes from the optic 

 vesicle. The outer chamber of the eye of the squid is never com- 

 pletely closed. In the vertebrates it is never open to the exterior, 

 but forms by a splitting of mesenchyme outside of the lens. The 

 iris in the vertebrate eye is inside of the sclerotic layer, while in the 



.^-f 



Fig. 65.— The vertebrate eye (human), vertical section in situ, a, eyelash; 

 b, lid; c, bony orbit; d, e and g, muscle.s;/, optic nerve; h, anterior chamber 

 filled with aqueous humor; i, pupil; j, conjunctiva, a transparent membrane 

 continuous with the lining of the eyelid; A:, cornea; I, iris; m, lens; n, sus- 

 pensory ligament of lens; o, retina; p, choroid coat; q, sclerotic coat; r, mu.s- 

 cles to ligament suspending lens; s, posterior chamber containing vitreous 

 humor; t, point of entrance of optic nerve; u, fatty tissue. (From Woodruff.) 



squid it is a projection from the margin of this layer. They are 

 remarkable analogous organs, and an e.xcellent example of con- 

 vergence. 



Blood. In the blood of the same three classes a similar diversity 

 prevails. Insect blood consists of a fluid plasma in which float 

 leucocytes similar to those of the vertebrates. The blood does not 

 carry oxygen to the tissues, however, and there are consequently 

 no red corpuscles. Oxygen reaches the tissues through the inde- 

 pendent tracheal respiratory system. The blood of the mollusca, 

 unlike that of the insects, must carry the animal's supply of oxy- 

 gen, hence it contains a blue pigment, haemocyanin, which con- 



