138 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



As far as my observations go these eyes are somewhat like the 

 dorsal eyes of Onchidium, and not of the vertebrate type ; that 

 is to say the nerve is not spread out on the inner surface of the 

 retina and the rods or cones are not reversed. I take this 

 opportunity of saying that I do not consider the negative 

 conclusions of any observer as decisive as to the character of the 

 organs to which I am referring on the mantle. I have known 

 many able and experienced histologists, possibly from an excess 

 of caution, unable to find eyes in the mantle of Onchidium, 

 and this certainly not for want of any manipulative skill in 

 preparing the sections. 



Amongst the Annulata we find something very similar to what 

 I have described above. Eyes are placed in large numbers on 

 certain organs in their young stage and subsequently change their 

 position. Among the Chcetopoda, the eyes which are present in 

 the larva, and even in later stages, disappear or are represented 

 by mere pigment-spots. 



Operculum Eyes. — The character of these eyes depends upon 

 whether the opercula are calcareous or chitinous. In the former 

 case small glassy tubercles, pedicels or bosses stud the surface of 

 the operculum. The manner in which these occur is so varied 

 that separate details will be required for each species. As a type 

 I may take the genus JVerita, the shell of which and the columella 

 are studded with small, oblong, transparent bosses or rounded 

 tubercles. Under the microscope these are found to be penetrated 

 by smaller sense-organs, but there are solitary eyes as well 

 either on raised calcareous pedestals or spherical, sessile eyes. 

 These are also found on the chitinous opercula associated with 

 minute spherical highly refractive bodies, which are like eyes, but 

 hitherto have not satisfactorily shown the minute interior structure 

 which has been detected in other visual organs. 



However extraordinary these sense-organs, with a double office 

 may seem, we are familiar with such a state of things amongst the 

 Annulata. The abundance of eyes with which the Platyhehninthes 

 are furnished, agree in so remarkable a manner with the structure 

 of the organs of feeling, that a condition appears to exist in which 

 specific sensory organs are evolved from mere organs of sensation 

 found in the integument. (See Gegenbaur, Comp. Anat. Vermes 

 Sect. 125.)* 



*" Non-zoological readers, when dealing with the genera of the lower 

 sub-kingdoms, must put aside the ideas formed by visual organs of the 

 ordinary type, as the animals have eyes both in number and variety of 

 structure widely departing from the usual acceptation of the term. The 

 following quotation from Gegenbaur in treating of the visual organs of 

 Vermes will show what extraordinary variations we may expect to meet 

 with. Speaking of the way in which the nerves are pressed together 



