DEVELOPMENT OF THE PECTINIBRANCHIATA. 335 



pearance here and there upon the upper edges of the rounded 

 lobes, and in a short time spread over their whole surface 

 (figs.5 &, 6) ; the lobes are then rounded and provided with cilia, as 

 well as with cirrhi (fig. 15). After the formation of the lobes 

 and of the foot, we see a semi-transparent granular substance 

 between the grouped ova and the membrane which invests them, 

 exuding and becoming the rudiment of the mantle, covering 

 itself superficially with a membrane which occupies a greater or 

 less space and takes on a determinate structure (fig. 4). Sub- 

 sequently we observe, on the lowest part of the mantle, a hemi- 

 spherical transparent body, which is the rudiment of the shell 

 (figs. 5, 6, 7«)» The foot increases in size and takes on a more 

 rounded form, and at its base we distinguish early the two audi- 

 tory organs (figs. 7? 8, 9^) . They are constituted by two spherical 

 vesicles, pellucid as water, filled with a perfectly transparent 

 and colourless liquid, and early exhibiting a double contour. 

 Each vesicle encloses but a single otolith. When the animal is 

 placed under the compressorium these organs become very di- 

 stinct, and by increasing the pressure, each otolith readily be- 

 comes separated into four regular segments. The vibratory 

 movement which most authors have observed in these otoliths, 

 has not been observed by us in those of Buccinum undatum, neither 

 have we been able to distinguish any cilia upon the internal 

 surface of the vesicle, although we have employed the strongest 

 magnifying powers. 



The eyes probably appear at the same time as the auditory 

 organs, for we have never seen the latter without being able to 

 observe the former. Leydig has remarked, that the rudimentary 

 eye is a vesicle which exists at the root of the tentacles. We 

 have confirmed the exactness of this observation; the internal sur- 

 face of this vesicle is always provided with cilia (fig. 10 a). It is 

 filled with a liquid, enclosing a quantity of pigment- granules, of a 

 strong, clear yellow colour, which are invested by an excessively 

 fine pellicle (fig. 10 b). At this period the vibrating cilia com- 

 municate a rotatory movement to these granules. We have 

 been unable to distinguish any lens at this period ; it is only 

 subsequently developed. At the same epoch as that in which 

 the eyes appear, we also found the two conical tentacles and 

 the rudiments of the salivary glands ; the latter always make 



