50 DEVELOPMENT. 



lost. The velum, that at first surrounded the fore-part of the 

 body is more elevated to the dorsal side after the appearance of 

 the mouth, since the mouth is not placed in the middle of the 

 velum, but beneath its narrow portion, and is dorsall}^ o\erhung 

 by the velum, whilst venti'ally the foot extends be3ond it. The 

 body becomes more elongate, and soon at its posterior dorsal 

 portion the delicate, hyaline, cup-like shell appears, in which 

 also a distinct laminated structure may be detected ; at the pos- 

 terior part of the foot the operculum appears at the same time. 

 The sense organs are now developed ; at each side in the velum 

 the tentacle arises as a papilla ; internally by the side of the 

 oesophagus the auditory vesicles arise, the otoliths then appear 

 therein ; and immediately thereafter, or at the same time, along- 

 side of the tentacles the eyes are formed, which, at first, like the 

 organs of hearing are vesicles, lined with cilia internally, the 

 lens appearing later. The central portions of the nervous sys- 

 tem, the ganglia around the oesophagus, now become visible. 

 Along the border of the shell the body wall is raised into a 

 ridge, the beginning of the mantle, and, as the shell grows far- 

 ther forward, the intestine in most prosobranchiates (not in 

 Chiton), instead of terminating posteriorly, begins to be pushed 

 forwards, so that the anus is likewise advanced with the border 

 of the shell to the right side of the body. 



The pharynx now appears as a distinct jiortion of the animal 

 and within it, the different parts of the lingual membrane may 

 be distinguished as the middle, inter, and lateral plates, and 

 according to Troschel's observation, the genus to which the larva 

 belongs may be already determined by the teeth. One may 

 already notice the commencement or the spiral winding of the 

 shell, and within it is contained one loop of the intestine and 

 many large cells or yolk-spheres, which become the liver. 



As soon as the anus commences to be pushed forwards and the 

 intestine becomes a distinct canal, the body cavity begins to 

 appear with blood in it. There is as yet nothing to be seen of 

 the heart, and the circulation of the blood is effected by the con- 

 traction and dilatation of the hollow foot, or often by means of 

 an elevation on the neck, consisting of a meshwork of fibres, the 

 cervical vesicle. By means of the heart, as soon as it is devel- 

 oped, the fluid in the body-cavity, the blood, is put into motion. 



