ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 169 



problem is to discover which cells are in connection with the fibres of 

 the optic nerve, or which exhibit a " rod " structure. Hesse finds rod- 

 like structures in all the Gastropod eyes which he has studied, and he 

 gives a brief account of their structure, which exhibits considerable 

 diversity, in Helix, Limax, Patella, I'urbo, Murex, and other forms. 

 In Helix and Patella he demonstrated the expected connection between 

 the rod-bearing cells and nerve-fibres. Between these truly optic cells 

 there are always indifferent cells without rods. 



As regards the distribution of pigment in the retinal cells, "all 

 possible combinations are realised." Rod-cells and indifferent cells may 

 be pigmented, as in Pleurobranchus and Murex ; the rod-cells maybe 

 pigmented and the indifferent cells not, as in Patella ; the converse is 

 true of Helix and Turbo ; or both rod-cells and indifferent cells may be 

 free from pigment, as in Limax. The absence of pigment in Limax 

 shows that the pigment has no essential role in vision ; it simply serves 

 to isolate the optic elements, and may be dispensed with altogether. 

 In Limax there is an accessory retina, or Nebenretina, comparable to 

 that which Brauer has described in deep-sea fishes. 



Structure of Lucapina crenulata.* — J. F. Illingworth gives an 

 account of this mollusc, and notes the following results as most impor- 

 tant. The epipodium is rudimentary and the nerve going to it is very 

 weak. The pharynx is enlarged into a crop-like pouch with many 

 folded digestive glands. Three large distinct hepatic ducts enter the 

 stomach. 



The nephridia are very unsymmetrical. Both have external openings, 

 and the right has an indirect reno-pericardial duct. The oviduct opens 

 just within the external papilla of the right kidney, and the reno-peri- 

 cardial duct is a short tube leading from the right side of the pericardial 

 cavity, and opening into the oviduct. The epithelial cells lining the 

 duct are very large, with exceedingly long cilia. 



The shell muscles are distributed along the margin of the shell and 

 very weak. 



The vascular system is closed ; there are two auricles with a ventricle 

 between them ; joining the ventricle is a large, rectangular aortic cham- 

 ber, from which three aortas arise — an anterior or buccal, a gastric, and 

 a, posterior or genital aorta. The mantle circulation is well developed ; 

 the blood is distributed by a pallia! artery that surrounds the body and 

 returns in the pallial sinus, which lies parallel and close to the artery. 

 The blood does not pass through the gills before returning to the heart. 

 The pedal collectors form a close meshwork of veins over the inner 

 surface of the foot. The ctenidia are symmetrical. 



The cerebral ganglia are joined to" the pleuro-pedal ganglia by two 

 pairs of connectives. The pleural and pedal ganglia are in the form of 

 short cords closely fused along their whole length. A ganglionic nerve 

 lies just within each osphradium. The circum-pallial cord, a chain of 

 small ganglia, encircling the visceral cavity, is joined to the pleural 

 cords by a great number of connectives, each of which sends a small 

 nerve to the epipodium. 



* Zool. Jahrb., xvi. (1902) pp. 449-SO (3 pis. and 15 figs.). 



