MOLLUSCS. 765 



Most molluscs 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 in 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 molluscs 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 (velum] 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 1 . 



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 antennae has been observed by SPAL- 

 LANZANI, SENEBIER, and BONNET 2 . 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 3 . 



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. u. fig. i, 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. VOGT, 

 sur Vembryogenie des Mollusques gasteropodes, Ann. des Sc. nat., 30 Se*rie, Tom. vi. 

 1846, pp. 5 90, PI. r 4, where, besides, the earlier observations are noticed histo- 

 rically. 



2 See of the last-mentioned CEuvres d'Hist. natur. et de Philosophic, 8vo. XI. 1781, 

 pp. i 61. 



3 SCHWEIGGER NaturgcscTi. der SMettl. ungegl. Thiere, s. 629, 630. 



