ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 251 



period in winter at the same temperature. The difference is probably 

 referable to the presence of the epiphragm in winter. Emergence from 

 the shell is the result of external conditions of moisture and tempera- 

 ture, and may be brought about at any time, though more slowly in 

 winter than in summer. The snail is able to absorb considerable quan- 

 tities of water through the skin, but absorption from a damp atmosphere 

 does not take place. 



Structure of Vitrina.* — Ernst Eckardt has studied the structure 

 of various species of Vitrina — small snails with somewhat degenerate 

 shells. Ciliated epithelium occurs on the sole and at the spiracle ; there 

 are two kinds of mucus-glands in the skin ; there are two special glandular 

 areas — one corrresponding to the " Nackendrtise " of Buliminus, the 

 other around the spiracle ; there are glandular cells with homogeneous 

 secretion among the epithelial cells of the roof of the canal of the pedal 

 gland. 



There is only a slight development of ostracum and hypo-ostracum ; 

 the peri-ostracum is formed in the mantle-groove ; there seems to be a 

 secretion of the calcareous shell from the whole surface of the intestinal 

 sac. The surface of the shell in the embryos shows pores which after- 

 wards disappear ; there is no lime in the embryonic shell. There is 

 internal pigmentation even when there is none in the skin. 



" Semper's organ " is not a definite organ ; it consists mainly of the 

 ganglia of the oral lips. There is a richly innervated transverse ciliated 

 groove on the roof of the mouth, which is probably gustatory and tactile. 

 The cells of the supporting epithelium of the radula share in forming 

 the sub-radular membrane. Twelve odontoblasts form the rachis-tooth 

 in Vitrina brevis, and ten the lateral tooth. 



There are two kinds of cells in the salivary glands. The calcareous 

 cells of the liver serve secondarily for absorption, which also occurs in 

 the mesenteron. It seems that the gut is shorter in the mountain 

 species, which are in part carnivorous. 



The optic nerve is an independent nerve. The buccal nerves are 

 poorly developed. The most anterior pedal nerves are probably con- 

 cerned with touch. The penis nerve arises high up on the pedal. There 

 is no trace of olfactory organ. The lens of the eye is probably formed 

 from Beck's cells. The statoliths are formed by the vacuole cells of the 

 statocysts from the fluid contained in them. 



By the torsion of the shell a transverse position of the kidney has 

 been induced. The podocyst appears to be a motor embryonic organ 

 connected with the circulation. Ureter and rectum open separately into 

 the respiratory passage. There is a typical urinary groove. The repro- 

 ductive organs have many peculiarities in detail, and particular attention 

 is devoted to the dart-sac and its gland. 



S. Lamellibranchiata. 



Insertion of Muscle on Shell.f — A. Briick has studied this in 

 freshwater mussels. The attachment of the muscle to the shell is effected 



* Jen. Zeitschr. Naturw., li. (1914) pp. 213-376 (1 pi. and 82 figs.), 

 t Zool. Anzeig., xlii. (1913) pp. 7-18 (5 figs.). 



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