ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 39 



Grenadier in the eyes of Heteropods and Cephalopods. In the Proso- 

 branchiata it is the unpigmented flask-shaped cells which present this 

 arrangement ; they closely resemble the retinal cells of the Heteropoda, 

 and have at their base a large nucleus which colours intensely with 

 carmine and hsematoxylin. The homology is not afl*ected by the fact 

 that the retinal cells of Heteropods contain pigment, for the cells in 

 Prosobranchs, which have been hitherto described as being devoid of 

 pigment, are not altogether so. The pigmented club-shaped cells rest on 

 the basal membrane by a filamentar stalk ; this is not of a nervous nature, 

 and these cells are not innervated and have no direct relation to the 

 perception of light. The rods are very difficult to see, as they are 

 destroyed by most of the reagents used for fixation ; they are best pre- 

 served by placing fresh eyes for from five to ten minutes in strong formic 

 acid, isolating pieces and teasing them carefully in a drop of water. 



In Nassa the zone of rods consists of closely packed delicate columns, 

 which are rounded off at their inner ends ; they are longest at the 

 fundus of the eye, and become shorter and shorter near the distal pole. 

 The rods project into spaces of the vitreous body, which are separated, 

 from one another by thin partitions. 



The connective framework discovered by Simroth in the vitreous 

 body and lens does not consist merely of filaments, but of numerous 

 stellate cells ; these have a nucleus which does not always colour in the 

 same way with carmine and hsematoxylin ; Patten • seems to think that 

 his retinophorje (the rod-cells) have two nuclei, but if so he has mis- 

 taken the nucleus of a stellate connective- tissue cell for the second 

 nucleus. The fibres from the rod-cells are not, as Hilger thinks, of a 

 nervous nature ; they do not end in the expansion of the optic nerve, 

 but in the basal membrane. 



The vitreous body is not, as has been generally supposed, com- 

 pletely structureless. If the pigment be removed by the action o 

 chlorate of potash and hydrochloric acid, sections will show that the 

 gelatinous mass has completely disappeared, and a plexus will be left of 

 fine fibrils, in which cell-nuclei are scattered; the fibrils are processes 

 of the cells to which the scattered nuclei belong. The vitreous body 

 consists, therefore, of connective tissue formed of cells with numerous 

 processes, and of a gelatinous intermediate substance. The lens has the 

 same structure. 



In his account of the eyes of Heteropods, the author confirms in 

 many points the description given by Grenacher, to which he makes 

 some additions. All the parts of the Gastropod eye are present in that 

 of Pecten ; but the retina is developed on the anterior side of the optic 

 vesicle in correlation with the position of a lens peculiar to the eye of 

 Pecten, which lies in front of the optic vesicle. 



5. Lamellibranchiata. 



Influence of Light.* — M. E. Dubois describes the retraction of the 

 siphon of Pholas dactylus under the influence of a beam of light. Even 

 detached from the animal the siphon keeps this power for several days. 

 The siphon as a whole is impressionable by light ; the sensory struc- 

 tures must be diffusely scattered. The author has made numerous 

 experiments on the relation between the muscular contraction of the 



* Comptes Kendus Soc. Biol., v. (1888) pp. 714-6. 



