368 INVEETEBEATA CHAP. 



centre, then the cells beneath this thickening disappear and it is added* 

 to by the more lateral cells. 



If this description be followed it is clear that the. primary eye 

 vesicle, with its contained inner segment of the lens, corresponds to 

 the eye as we find it in Gastropoda; and that the outer segment of 

 the lens and the outer chamber of the eye are subsequent additions. 

 The ciliary body consists of the adpressed posterior wall of the outer 

 chamber and anterior wall of the inner chamber of the eye. 



The hinder wall of the primary eye vesicle forms the retina. 

 This consists at first of a single layer of columnar ectodermal cells 

 with the nuclei at different levels, bounded externally by a basement 

 membrane. Laterally it is continuous with the layer of small cells 

 which forms the lens. Soon the single layer constituting the retina 

 changes into many layers of small rounded cells ; of these the outer 

 layers begin to pass outwards through the basement membrane, and 

 they constitute the nervous layer of the retina. From the innermost 

 layer of cells visual rods grow out (y.c, Fig. 300), pointing into the 

 cavity of the eye-sac; but these cells do not all undergo this transforma- 

 tion; alternating with the visual cells are cells which secrete pigment. 

 The inner portions of the visual cells, that is, the portions turned 

 towa'rds the cavity of the eye-sac, and these pigment cells, alone retain 

 their primary position with regard to the basement membrane. The 

 nervous portion of the retina is thus seen to consist of two layers of 

 nuclei with a clear space between them (almost certainly occupied by 

 dendrites of the nerve cells), and the whole presents a striking analogy 

 to the layers of cells in the human retina, except that the layers occur 

 in the reverse direction so far as the incidence of light is concerned. 



GENERAL CONSIDEKATIONS ON THE ANCESTRAL HISTOKY OF MOLLUSCA 



When we review the account of the development of Mollusca 

 given in this chapter, certain facts stand out clearly. First, in the 

 early larvae of Patella, Dentalium, and Dreissensia we are evidently 

 dealing with a single type, and this type must be classed as a Trocho- 

 phore larva, similar in all essentials to the Trochophore larva of 

 Annelida. Therefore the common ancestral group from which 

 Gastropoda, Scaphopoda, Pelecypoda (and we may add Solenogastres) 

 spring, must have had a Trochophore larva. In a word, all Mollusca 

 are thus shown to be descended from an ancestor represented by the 

 Trochophore, i.e. the same ancestor as gave rise to the Annelida. 



What, it may be asked, was the factor which caused two families 

 of the descendants of this ancestor to diverge so widely from one 

 another in structure ? We must surely look for this factor solely in 

 a divergence of modes of life. Now, the fundamental type of habits 

 common to all Annelida is a burrowing mode of existence ; and from 

 that, coupled with a wriggling method of locomotion during their 

 occasional excursions into the upper water, we were able to deduce 

 the main peculiarities of their adult structure. But the habits of 



