Development of the Pectinibranchiata, 359 



the same time as the auditory organs ; but, according to the in- 

 vestigations which have since been made, this is not the case, 

 for the eyes can only be perceived a httle while after the form- 

 ation of the auditory organs. Leydig has pointed out that at 

 first the eye is a vesicle occurring at the base of the tentacles. 

 We have had the opportunity of confirming his remarks, but we 

 have also found that the inner wall of this vesicle is furnished 

 with cilia. This vesicle is filled with a liquid, in which there 

 is a multitude of strongly- coloured, yellowish pigment-grains, 

 which are surrounded by an extremely delicate pellicle. When 

 the cilia acted upon the pigment-grains, the latter acquired a 

 rolling motion. We were unable to observe the lens ; it does 

 not appear until a later period of development. In observing 

 the eyes, we also saw the two conical tentacles. 



As we have already stated, the pharynx is one of the first 

 organs to make its appearance; some time afterwards, the pro- 

 boscis, the stomach, and the oesophagus show themselves. The 

 latter makes its appearance as a cylindrical cavity surrounded 

 by the proboscis, and in its excessively delicate walls we perceive 

 several very clear lines, which are the earliest-formed muscular 

 fibres. As soon as the oesophagus issues from the proboscis, it 

 bends a little backwards and upwards, follows the lower part of 

 the proboscis for some time, then again describes a curve by 

 bending a little to the left, and enters the stomach. It is very 

 difficult to follow the oesophagus during its development, as it 

 is not only surrounded by the proboscis, the walls of which are 

 thicker and less transparent, but it is also completely covered 

 by it. This is the reason of our being unable to decide whether 

 the whole length of the oesophagus is formed at once, or whe- 

 ther it becomes elongated by descending towards the stomach. 

 The latter appears at first almost in the form of a ball, and 

 seems to be formed by a single vitellus secreting a greyish, serai- 

 transparent mass, which solidifies and forms a delicate mem- 

 brane, which is first of all produced upwards and unites with 

 the oesophagus, and afterwards downwards to form the rudi- 

 ments of the intestines, which curve to the right, there form a 

 curvature, pass to the opposite side, and lastly terminate by an 

 anus in the branchial cavity (fig. 3 t). The stomach is always 

 filled with a crowd of vitelline granules, which are continually 

 rolling : this movement is caused by cilia, with which the whole 

 of its inner surface is covered. It is not only the inner wall of 

 the stomach that is clothed with cilia, but also that of the oeso- 

 phagus and the whole intestinal canal. 



It is only now that we observe the first traces of the nervous 

 system, which are recognizable by two oval, yellow, and com- 

 pact bodies (cerebral ganglia) surrounding the oesophagus. At 



