ANATOMY AND LIFE HISTORY OF MOLLUSCA. 171 



In the centre of each a rod Stabchen is situated. This has a 

 conical shape, is attached with a broad base to the bottom of the 

 concave space, and tapers rapidly at first, and more gradually 

 afterwards to a fine point. Its faces appear concave. The upper 

 pointed end of the rod is continued in the shape of a very fine 

 thread, extending through the centre of the hexagonal cell and 

 joining the ganglia cell-layer. 



Nothing is to be added to Semper's statements regarding tho 

 nervi optici. 



"It is remarkable that the Onchidium damelii never retracts 

 her tubercles or feelers, however near the forceps or scissors 

 approach them, until they are actually in contact. This might 

 lead one to assume that this animal is far-sighted. The concave 

 lenses on the upper ends of the facets, below the large spherical 

 main lens appear as a secondary arrangement produced for the 

 purpose of counteracting the bad effect of an oval or spherical 

 lens in air. The lens was originally, probably, adapted for seeing 

 in water, and therefore had such a great curvature and short 

 focus. When the Onchidium took to living on land this lens was 

 too strong — for use in air, — and then the little concave cells 

 might have been produced to counteract the excessive power of 

 the main lens." 



Having had the advantage of studying a fine series of sections 

 of the Australian Onchidium, kindly made for me by Mr. 

 Whitelegge, who made those studied by Dr. Lendenfeld, I am 

 able to record my own observations on the eyes and the general 

 anatomy of this most interesting species. In doing so I shall 

 enter into the matter a little more fully, as 1 am sure that the 

 observations of Dr. Semper are very little known in this country, 

 or perhaps anywhere except in scientific circles. 



It has been already remarked that Onchidium is a genus of 

 Mollusca without any shell. On making a section through the 

 skin, however, this is found not to be strictly true. The little 

 papillae which so thickly cover the surface with warty excrescences 

 are, on the dorsal area, covered with a layer of little cavities, in all 

 of which there is a much smaller brown limestone concretion. 

 Inside the layer there are sometimes larger cavities, generally of an 

 ovoid shape, in which there is also a rounded calcareous concretion. 

 These are the otolith-like concretions. They are numerous and 

 of relatively large size. In pi. vii., fig. 9, is represented a section 

 through one of these layers of rudimentary shell, which is> 

 interesting for many reasons. First of all there is a layer of skin 

 between the outer surface and the concretionary cells ; and 

 secondly these cavities have generally a rounded outline, while the 

 calcareous matter is rugged and angular. Another point of 

 interest is that some of these concretionary nodules extend far 



