196 ]M. E. Claparede on the Development of Neritina fluviatilis. 



XX. — On the Development of Neritina fluviatilis. 

 By E. Claparede*. 



The capsules, which are usually taken for the eggs of the Neri- 

 tincBy are round balls, n little flattened on one side, 07— 1 mill, 

 in diameter, enclosed in a hard shell, which has been described 

 as calcareous, but which does not effervesce with acids. They 

 consist of two segments, firmly united at first, but readily sepa- 

 rable afterwards ; the upper one is larger, and forms a hemi- 

 spherical dome ; the other is of a flatter form, and resembles a 

 bowl. Each female attaches her eggs to the back of her neigh- 

 bour, but not to her own. In the neighbourhood of Berlin this 

 is not the case, but the capsules are usually attached to stones, 

 or, where these are wanting, to the shells of Dreissena and other 

 Mollusca. When the capsules dehisce, the lower segment re- 

 mains attached, and this has given rise to an erroneous opinion 

 that the capsules corrode the surface of the shell, producing 

 small indentations : this appearance is caused by the raised 

 margins of the lower segments. 



The capsules consist of two membranes, not easily separated : 

 the inner one is delicate, perfectly colourless, transparent and 

 structureless ; the outer thick, yellow and opake. The outer 

 membrane of the upper segment has sometimes an appearance 

 of cellular structure, which is due to adherent Diatomacese : the 

 outer membrane of this part exhibits no recognizable struc- 

 ture ; that of the lower segment, however, shows a reticulated 

 structure, caused by round or oval spaces separated by darker 

 intervals. These are not cells, but lighter and probably thinner 

 spots in the capsule. Their diameter varies from ()'006 to 

 0-04 mill. 



The margins of the segments possess a horizontal border, like 

 the rim of a plate, 006 mill, in breadth ; these two borders are 

 in contact. Their surfaces are finely striated or furrowed, and 

 the adherence of the segments is doubtless caused by the raised 

 lines on the rim of one segment fitting into the furrows of the 

 other. 



In the earliest stage of development observed, the capsules 

 contained from forty-five to sixty or more spheres. These 

 might have been globules of segmentation, as only a single em- 

 bryo was developed in each capsule ; this was originally very 



* An abstract of part of M. Claparede's paper " On the Anatomy and 

 Development of Xeritina fluviatilis," in Miiller's Archiv, 1857, pp- 109-248. 

 This abstract is confined to the portion uhich refers to the earhest develop- 

 ment of Neritina, as this alone bears upon the opposite views of the de- 

 velopment of these Mollusca held by Dr. Carpenter and MM. Koren and 

 Danielssen. — W. S. D. 



