196 SUMMARY OF CURRENT RESEARCHES RELATING TO 



" hermaphrodite gland " contained only spermatozoa, but the author is 

 not certain whether this is a case of protandry or of separate sexes. 

 Herr KoUmann has erroneously stated that the arterial system of 

 Aplysia is completely closed, for the capillaries were found to communi- 

 cate with intermuscular lacunae or with the general cavity. The purple- 

 gland plays a very active part in the depuration of the blood and in the 

 elimination of substances which are hurtful to the animal ; if methylen- 

 blue be injected into a living specimen the glandular capsules of the 

 gland will be found gorged with this substance. 



The Heteropod Eye.* — In the second of a series of papers on the 

 comparative anatomy of visual organs, Prof. H. Grenacher describes the 

 eye of Heteropoda, and specially that of Pterotrachea coronata Forsk. 

 He sums up his conclusions as follows : — 



(1) The retina of the Heteropoda, like that of the Cephalopoda, is 

 not to be considered as made up of histologically distinct layers. It 

 consists of a single layer of cells, whose individual elements are made 

 up of nucleus-bearing portions, rod-sockets, and rods. The first named 

 lie outside, both the others inside a thin limiting membrane. 



(2) The striated or fibrillated contents of the nucleus-bearing 

 portions of the retina cell cannot be referred, with any sufficient ground, 

 to a disruption into nerve-fibres ; rather are the striations related to 

 the formation of the so-called radiculae, which, as root-like processes, 

 seem destined to fix the retina cells to the cuticle. 



(3) The rod-sockets, also finely striated, are segaents of varying 

 length, which is determined by the height of their point of insertion 

 in the rods above the bounding-membrane. 



(4) The rods must be considered as compound structures, since a 

 number of socket-parts are in connection with each of them, and indeed 

 because each rod owes its origin to a number of retinal cells. This 

 is also the case in the rhabdoms of the Arthropoda and Cephalopoda ; 

 but while the components of a rhabdom are placed side by side, they are 

 here in rows over one another, one end free, the other uniting with the 

 corresponding socket-parts. Their transverse striation, contrary to 

 M. Schultze's statement, is due to a relatively simple lamellar texture. 



(5) The rods are arranged in longitudinal rows (of which there are 

 six in Pterotrachea) which extend over the retina in nearly parallel 

 courses. 



(6) The retina is traversed by a cleft to the whole depth of the 

 rows of rods, running parallel to them, and separating them into dorsal 

 and ventral halves. In the dorsal half are two, in the ventral four 

 rows. The dorsal rows have their free side ventral, the ventral rows 

 are free dorsally. 



(7) The retina is innervated from a layer of nerve-fibres which run 

 between or under the basal ends of the retinal cells. They run to that 

 part of the retinal cells where these split up into radiculae, and there 

 unite with them. There is no ground for supposing that the nerve-fibres 

 pass into the retinal cells on the analogy of the Cephalopoda. Besides 

 the nerve-fibres, in the dorsal half, there are small ganglionic cells. 



(8) The structureless limiting membrane which extends between 

 the free ends of the rods and the vitreous body gives off", on the side 

 next the rods, rows of fine fibres which insinuate themselves between the 



* Abh. Nat. GeBell. Halle, xvii. (188S) pp. 1-64 (2 pis.). 



