ZOOLOGY AND BOTANY^ MIOEOSCOPY, ETC. 53 



Baltic and tliat of German (? Swiss) fresh-water lakes ; of course, however, 

 there are some species in the Baltic which are not, so far as is known, in 

 the lakes also. 



Microscopic Organisms in Fresh Water.* — Dr. G, Asper, in investi- 

 gating the pelagic fauna of fresh-water lakes, finds that the most abundant 

 forms in Lake Zurich are CeraHum, Dinobryon, Volvox, Vorticella, Anurea, 

 Polyarthra, and Synchseta. The Protozoa and Eotifera considerably exceed 

 in numbers the Entomostraca ; and of the above forms, one genus will be 

 predominant at one time, whilst at another time some other genus will 

 exceed it in numbers. Pelagic species of Difflugia, and rich development 

 of diatoms, especially Asterionella formosa, were also found. 



Amphibious Life in Rhizomorpha.t — Dr. E. Schneider has made an 

 investigation into the animals found living amongst the mycelia of 

 Mhizomorpha suhterranea in a damp grotto near Dresden ; he found 51 

 forms, 24 of which are Protozoa, 8 Vermes, 18 Arthropoda, and 1 

 Mollusc ; of these 30 are typically aquatic, 14 terrestrial, and 7 amphibious. 



MoUusca. 



Eyes of Mollusca.| — ^Mr. W. Patten communicates the results of his 

 investigation of the eyes of Molluscs (and Arthropods §), His results differ 

 widely from those of Grenacher and other authorities, and lead to the 

 reduction of the essential parts of all visual organs to one structural plan, 

 which can be followed throughout the whole animal kingdom. Some 

 difference of opinion exists as to the author's views. || 



1. Area. — The eyes of the timid Areas, which detect even a faint shadow 

 cast on the water, are of three kinds — facetted, invaginate, and pseudo- 

 lenticulate. The first are aggregate, and are confined to the anterior and 

 posterior thickenings of the mantle edge. The invaginate eyes, forming 

 oval pigmented cups, are smaller than the latter, and form a narrow band 

 along the summit of that portion of the ophthalmic fold of the mantle 

 margin, beneath the ventral opening in the shell through which the byssus 

 projects. The third form resembles the last type, but is not invaginated, 

 and consists of a few retinal cells, covered with a lenticular and refractive 

 body like a cornea, or lens. They occur irregularly among the invaginate 

 forms. There are altogether about 1300 eyes for each individual. 



After a brief historical review. Patten proceeds to describe the structure 

 of the compound eyes. They consist of 10-80 " ommatidia." Each wedge- 

 shaped ommatidium consists of a central core of two fused cells (retinophorse), 

 whose bases are directed outward, and support a double, highly refractile 

 rod or perceptive element. The central cells (or retinophorse) are sur- 

 rounded by two rows of four pigmented cover cells (or retinulse). The 

 outer end of each of these is capped by a thick layer of transparent and 

 perfectly homogeneous cuticula. The inner, membranous prolongations 

 consist of flattened, and longitudinally striated " hacilli" ending abruptly 

 in root-like fibres. The expanded pigmented cells form a broad collar for 

 the retinophorse. The connection of the nerve-fibres with the pigment cells, 

 and with the retinophorse is then noted. A special aggregation of fibres can 

 be seen passing along the outer surface of the latter, and they also inclose, 

 being double, a centre bundle. 



Tlie invaginate eyes are simple thickened portions of the hypodermis, 



* Arch. Sci. Phys. et Nat., xvi. (1886) pp. 366-7. 



t SB. Preuss. Akad. Wiss., xxxix. (1886) pp. 883-900 (1 pi.). 



X MT. Zool. Stat. Neapel, vi. pp. 542-756 (5 pis.). § Cf. infra, p. 82. 



II See Prof. E. E. Laukester in Quart. Joiirn. Micr. Sci., xxvii. (1886) pp. 285-92. 



