MOLLUSCS. 765 
Most molluses are oviparous; only few, as Paludina vivipara 
and Clausilia ventricosa, are viviparous. Development from the 
egg begins with a cleaving or separating of the yolk, as has been 
already noticed by us im other classes of animals. Presently the 
yolk begins after a longer or shorter period to rotate round an 
axis, which at first occurs more feebly, afterwards more rapidly and 
regularly. With this rotatory motion a progressive one is connected 
by which the yolk describes a circle. These motions are caused 
by cilia. It is to be remarked, that, according to the observations 
of Sars, LovEn and others, young marine molluses universally, 
though they be afterwards naked, possess a shell which like that of 
Nautilus is volute and covers the hind part of the body. Also 
these molluscs undergo a remarkable change of form, since, before 
tentacles appear, a large thin membrane, divided into two lobes, 
surrounds the mouth. This part (velwm) is surrounded by cilia, and 
is the principal organ of motion. In some molluscs this organ is 
found to be persistent, most conspicuously in Tethis, where the fine 
cilia surrounding the margin are replaced by cirri’. 
The power of restoration is in some species of this class very 
great. In various Helices the reproduction of the head that has 
been (partly) excised and of antenne has been observed by Spat- 
LANZANI, SENEBIER, and Bonner’. That the entire head grows 
again, as the experiments of SPALLANZANI appeared to indicate, may 
however be doubted, since the anatomical investigation of such 
snails as had been preserved by this observer in spirit afterwards 
proved that by the excision the first or cerebral ganglion had not 
been removed’. 
The nervous system of molluscs presents various degrees of 
development in the different orders and families. It may be re- 
garded as the general type, that there is a central mass above and 
(p. 135), and it is not apparent from his figures why he considers Tab. 11. fig. 1, to 
represent a female, fig. 2 a male individual of Helix pomatia. 
1 For the sake of brevity it must suffice that we cite the investigations of C. Voar, 
sur Vembryogénie des Mollusques gastéropodes, Ann. des Sc. nat., 3e Série, Tom. vt. 
1846, pp. 5—9go, Pl. 1—4, where, besides, the earlier observations are noticed histo- 
rically. 
2 See of the last-mentioned (uvres d’ Hist. natur. et de Philosophie, 8vo, x1. 1781 
pp. 1—6r. 
3 ScHWEIGGER Naturgesch. der Skelettl. ungegl. Thiere, s. 629, 630. 
5 
