Development of the Pectinibranchiata. 365 



As soon as the rotatory organs and the foot are formed, we 

 see the otoUthes, the sahvary glands, and the nascent shell ; the 

 spheres of segmentation then become less dark, the embryo en- 

 larges, the foot becomes thicker, and in the interior we perceive a 

 movement of rotation, w^hich indicates the nascent stomach, from 

 which a tube, the pharynx, is soon produced upwards, rounded in 

 the form of a funnel. But whilst this takes place, the vitelline 

 mass has greatly diminished, at the same time that the spheres of 

 segmentation, which were previously compact and opake, ap- 

 pear to be less compact and more transparent. A tube is pro- 

 duced downwards from the stomach, which, however, soon stops, 

 and in which we see a rolling movement (the rudimentary intes- 

 tine). The rotatory organs increase greatly in size, giving the 

 embryo a very brisk motion. The foot becomes thicker and 

 the pharynx firmer, and whilst all this is taking place, the vitel- 

 line mass diminishes to such an extent, that at last we can only 

 distinguish a few traces of it. The embryo is, properly speak- 

 ing, perfectly transparent, and thus acquires an airy appearance, 

 which, coupled with its very brisk movements, renders observa- 

 tions very difficult. Some time then elapses without any fresh 

 changes being observed, no new organ makes its appearance, 

 and the last traces of the vitelline mass disappear entirely. But 

 from this time we observe a retrograde tendency ; the embryo 

 becomes smaller and the foot thicker, whilst the rotatory organs 

 shrink, and the part where the shell is situated becomes round, 

 so that the whole acquires the form of a balloon. As the organs 

 disappear, the vital force gradually diminishes, and at last we 

 have before us a little shrivelled monster, which exhibits a slight 

 ciliary movement, but which otherwise remains quiet at the bot- 

 tom of the vessel until it dies. This is the case with all the 

 embryos which are developed from a single q^^ ; they have but 

 a short existence, during which only a few organs are formed. 



There is not much difference in the development of those 

 which are evolved from two eggs. Thus we have observed that 

 where two eggs unite, the vitelli undergo no segmentation, but 

 a rapid exudation of a greyish and semitransparent mass takes 

 place. This mass becomes firm, and converted into a membrane, 

 on which cilia are formed. The embryo is then formed in the 

 same way as that which is developed from a single q^^, except 

 that it is larger and stronger. When the foot, the rotatory or- 

 gans, the salivary glands, the stomach, the oesophagus, and the 

 intestines are formed, we see that a portion of the vitelline mass 

 is used up, and we then perceive a slight contractile movement 

 in the direction of the rotatory organs, which indicates the 

 future heart. This takes more and more the form of a clear 

 vesicle, in w^hich we detect some muscular tubes. The contrac- 



