ZOOLOGY. 455 



There is no ommatopliore ; the optic nerve is also not developed, and 

 the lens and vitreous body are equally deficient. The eye of this form 

 is therefore considered to represent an embryonic stage in development. 



In Haliotis the eyes are large and (as in Patella) there is an open cupj 

 the cells pass from one to the other. Pigment is only found around the 

 orifice of the cup. Ketractile ommatophores support the eyes; optic 

 nerves are developed of a remarkable character ; each divaricates into 

 two or three branches before entering the eye,' and the latter expand and 

 inclose the entire eye, coming into direct connection with the retinal cells. 

 A lens of gelatinous substance is found and a vitreous body occupies a 

 large part of the eye cup. The eyes are invested in loose connective 

 tissue, and there is no sclerotic nor any other investing membranes. 



In Fissurella the eyes lie just below the epidermis and the corneal 

 cells are separated from the epithelial by a delicate layer of connective 

 tissue. The "retinal cells belong to two groups; they may be long and 

 very delicate at their lower ends, or they may be broader and thicker 

 and more closely granulated, and the latter are moreover destitute ot 

 pigment. These thicker basal cells are regarded by the author, not as 

 supporting cells of the true retinal elements, but as those organs from 

 which the lens and vitreous body are developed ; the pigmented cells 

 alone function as the end-organs of the optic nerve." 



In fine, the author's investigations have led him to conclude that the 

 eyes of moUusks begin their development by an invagination of the 

 epidermis which is originally open to the exterior; as this becomes shut 

 off, the retinal cells become developed out of the epidermal cells. The 

 eyes of Patella present the simplest known condition. " In Nautilus the 

 eye is likewise oi)en to the exterior, and in the Hirudinea [leeches] we 

 may find organs of a somewhat similar construction. In cases of this 

 kind the use of the term retina should be avoided, and be replaced by 

 that of rod-cells. The cells which appear to be the organs for the per- 

 ception of light are very characteristicallj^ developed in the MoUusca; 

 what is here seen almost in diagram is found more or less distinctly in 

 all other mollusks. The so-called -retina consists of a series of elon- 

 gated cells, the anterior portion of which is filled up by dark pigment. 

 This pigment is more or less marginal in position, so that there is in the 

 center an unpigmented cylindrical canal which passes directly into the 

 unpigmented part of the cell." (J. R. M . S., (2,) 1, 724-725, from Z. W. Z., 

 XXXy, 461-478.) 



Tlie Neomenice and Chcetodermw. 



One of the most interesting groups of Invertebrates is that represented 

 by the genera Chaetoderma, Keomienia, and Proneonifeuia. 



The representatives of this type have been associated with very 

 diverse groups by various authors ; by some with the worms, by most 

 with the mollusks; and in the branch of mollusks, by some with the 

 Nudibrauchiates; but by the majority of anatomists with the Chitons. 

 Although few in number and of rare occurrence, or rather only found by 



