ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 197 



rods. They are most probably united with the cellular elements which 

 lie between the nucleus-bearing portions of the retina cells, and whose 

 distribution, in the ventral half at least of the retina, exactly resembles 

 that of the fibres. These cells are probably, as in the Cephalopoda, 

 the material for the formation of the fibres, and also for the limiting- 

 membrane itself. A further net of circular fibres arises apparently from 

 cellular elements which lie in the retinal cleft. 



(9) The nervous layer of the retina extends over this, out into the 

 pigmented epithelium of the so-called costal region, where it can be 

 followed with gradual diminutions to certain areas marked by epithelial 

 projections. On the ventral side of the eye, the fibres can he traced to 

 large cells surrounded by the pigment-epithelium. This points to an 

 analogous condition on the dorsal side, where the said cells are smaller, 

 but there the point cannot be settled. Whether it can be shown that 

 these cells are of secretory function, it is impossible to determine. 

 The relations of the Heteropod eye to the visual organs of the other 

 Gephalophora, and the general morphology of the latter are then 

 discussed. 



Entocolax Ludwigii, Parasitic in a Holothurian.* — Dr. W. Voigt 

 gives an account of a remarkable Mollusc found by Prof. Ludwig living 

 parasitically on Myriotrochus Rinkii ; the specimen which carried the 

 parasite was collected in Behring's Sea. It belongs, in the author's 

 opinion, to the group of Prosobranchiate MoUusca, of which it must form 

 the representative of a special suborder to be called Cochhsyringia, 

 These may be defined as endoparasitic Gastropods which, when adult, 

 have the form of an acephalous naked tube, which by means of its knoblike 

 anterior end, penetrates into the body-wall of its host. At the tip of 

 the anterior end there is an orifice which leads into the oral invagina- 

 tion. This has neither jaws nor radula. There are no circulatory or 

 respiratory organs. The female gonads have a rudimentary efferent 

 duct, and a well-developed receptaculum seminis ; the ova are evacuated 

 by tearing the wall of the ovary. The female of Entocolax is as yet 

 alone known ; its body is 1 cm. long, tubular, narrower behind than in 

 front ; for a short distance behind its anterior end it forms a large sphere 

 3 mm. thick, which contains the ova. 



Moutk-parts of Ancylus fluviatilis and Velletia lacustris.f — Herr 

 J. Ulicny describes the jaw and radula of the above forms. The jaw of 

 Ancylus is a deep arch formed of about 100 plates, which are somewhat 

 rectangular, and have their long axis directed towards the mouth of the 

 arch. Their surface is granular, and they are fringed at the lower end. 

 The centre of the arch and its extremities have only one row of plates, 

 but in the middle of each side they are six or seven irregular rows 

 thick. 



In the jaw of Velletia, there are about 60 plates pointed at each end, 

 i. e. somewhat lancet-shaped, and finely striated. They form a single 

 row still more deeply arched than in Ancylus. No one plate can be 

 said to have a central position. At the sides they lie pointing down- 

 wards and outwards, thus touching one another laterally only. 



The radula in Ancylus is composed of some 140 transverse rows of 

 teeth, curved so as to be convex backwards. The central tooth is 



* Zeitschr. f. Wiss. Zool., xlvii. (1888) pp. 658-88 (3 pis.), 

 t Verb. Nat. Ver. Brunn, xxvi. (1888) pp. 120-4. 



