32 Prof. Miiller on the Development 



appear very soon to break up and become dissolved. The four 

 large spheroids are even yet present when the whole surface of 

 the yelk has become invested with a layer of transparent cells, 

 and when the moving cilia are developed upon these. The four 

 large spheroids thus remain in the interior of the yelk, and only 

 become more closely appressed, remaining otherwise unchanged 

 superficially. If however they be crushed at the time when the 

 cortical layer of the yelk is already developed and begins to ex- 

 hibit ciliary motion, we find in their interior a great number of 

 clear nuclei, and I enumerated as many as twelve or more of 

 these simply inclosed within the large spheroids. It follows then 

 that the division of the clear nucleus of a yelk-spheroid precedes 

 the division itself in our mollusks. 



The greater number of my observations relate to the period 

 when the mollusks contained in the sacs were almost perfect, and 

 possessed a spiral shell of one turn and a half, out of which they 

 could protrude themselves and into which they could retire. The 

 shell resembles that of Natica more than any other, but the aper- 

 ture is as long as it is broad. By the friendly aid of the Director, 

 M. Koch, I was enabled to examine extensively the mollusks of 

 the local fauna contained in the collection of the Zoological Mu- 

 seum of Trieste. The aperture of the shell is as large as the rest 

 of the shell, or larger. The columella is almost straight, and 

 thence the operculum has one edge more rectilinear. The shell 

 appears also to be umbilicated. The body of the mollusk is, for 

 the most part, made up of the richly- ciliated foot and head. The 

 foot is transversely notched in the middle, and thence consists of 

 two lobes, an anterior and a posterior, which carries the opercu- 

 lum. In the middle of the notch of the foot there is a kind of 

 papilla with an aperture, in which a ciliary motion is perceptible, 

 and which I can only interpret as an opening of the so-called 

 water- vascular system. Above the anterior lobe of the foot is the 

 mouth, which is covered by a peculiar, sometimes rounded, some- 

 times notched lobe. This lobe has much smaller cilia than the 

 foot, and stiff immoveable cilia or hairs ; whenever I observed 

 them, these large cilia were motionless ; like the lobes, they re- 

 minded one strongly of the cephalic velum of so many mollusk- 

 larvge, and perhaps at an earlier or a later period they are active. 

 In this stage the young mollusks move about but little in the ve- 

 sicles. Between the mouth and the foot there comes forth at 

 times a peculiar, generally-hidden lobe, which possesses only short 

 cilia, not larger than those upon the dorsal surface of the head. 

 In the head we see the two auditory organs, vesicles, containing 

 a constantly oscillating otolithe. Above this, upon the head, 

 there are two short projections, the future tentacles ; but no trace 

 of the eyes is to be seen. Within the shell is the respiratory 



