DORIS. 



end, and bending upwards, runs almost parallel to the oesophagus, and terminates at one side 

 not far from the base of the oral lobe and close behind an oval sac (k), probably the incipient 

 genitalia. The alimentary canal is ciliated within, and particles of matter may frequently be 

 seen rotating in the interior of the stomach, influenced by the action of the cilia. 



Besides the parts just described, there is always a rounded, yellowish, and opaque 

 mass (j), sometimes divided into two, which is situated within the spiral portion of the shell, 

 and in contact with the stomach. This, according to M. Vogt, is the liver ; and he is probably 

 correct. On each side, near the commencement of the oesophagus, is a rather conspicuous 

 circular vesicle (/), containing a distinct nucleus ; the vesicle and nucleus being very distinct 

 in Eolis coronata. They have been considered by most writers on the subject to be the 

 auditory capsule and otolithe ; a view first adopted by Sars, and in which we are disposed to 

 agree, having observed the enclosed nucleus or otolithe to vibrate within its capsule in the 

 larva of Aplysia. 



In the mature larva, the foot is of considerable dimensions ; and the operculum, which 

 is attached to its posterior surface, is so delicate and transparent, that much care is required 

 to observe it accurately. Under favorable circumstances it is seen to be circular, extending 

 considerably beyond the fleshy support ; and indistinct circular lines of growth may occa- 

 sionally be observed upon it.* We have failed to detect the operculum in Dendronotus 

 arhorescens, Eolis picta, and E. olivacea. Our failure, however, in these cases may have arisen 

 from the excessive tenuity and transparency of the parts. 



Besides the large, locomotive, vibratile cilia of the oral lobes, the action of which is under 

 the control of the larva, the whole surface of the exposed parts is covered with minute cilia, 

 whose vibrations never cease. These cilia are most probably for respiratory purposes, and 

 may also assist in bringing food to the mouth, as stated by M. Vogt. 



When the larva is at rest, the oral lobes are pulled back into the shell, and the opercu- 

 ligerous lobe or foot, being drawn down, brings along with it the operculum, which closes the 

 orifice; but when in action (figs. 14, 15), the whole of these parts project beyond the opening 

 of the shell, the foot lying back against the spire ; and the oral lobes inclining forward, 

 their cilia commence to vibrate, and the larva, with the mouth of the shell upwards, moves 

 through the water with lively action, sinking or rising, or advancing onwards at its pleasure. 



We have not succeeded in tracing the development of the larva into the mature 

 form ; but it is not difficult to understand how this change is effected. When the larva is 

 placed with the mouth of the shell downwards, the oral lobes in front (we are speaking 

 particularly of Eolis), the anal termination of the intestine, and the oval sac representing the 

 generative organs, will be found on the right side, close to the base of the oral lobe, and the 

 operculigerous lobe or foot will be seen to extend backwards in a medium position, occupying 

 the place of the crawling disc. Thus it is evident that the principal organs of the larva only 

 require to be slightly modified in form, and it is changed into the mature animal, the shell and 

 operculum being cast off, and the oral lobes either absorbed, or altered into a veil or oral 

 tentacles. 



This view of the change of the larva into the mature animal is apparently verified by the 

 investigations of Professor Nordmann, who has traced the transformations in a species closely 

 allied to our Eolis disjecta. 



* Circular lines of growth are perfectly distinct on the operculum of Aplysia. 



