MOLLUSC A. 



53 



masses of the liver (/) , and at gav is a mucous or albu- 

 men gland belonging to the generative apparatus, of which 

 the larger portion is hidden in the spire. Do not tear the 

 internal organs apart, but carefully skin the mantle from 

 the spire, and observe their relative positions and connec- 

 tions. After this they may be carefully separated with a 

 sharp knife. 



When this has been done, and the connection of the 

 alimentary canal and liver tubes traced, it will be easy to 

 follow the intestine with the scissors along its posterior 

 part, where it is imbedded in the wall of the pulmonary 

 sac, until it opens in the angle of the orifice. 



Cut away entirely the tube of the oesophagus or throat, 

 and the large muscles accompanying it. This will expose 

 the upper side of the mass of ganglia below, or the pedal 

 ganglion, as it is called. Close observation of the top 

 surface of this will exhibit a peculiar disc or space filled 

 with yellowish granules. This is the rudiment or essential 

 part of an organ of hearing ; namely, a sac filled with small 

 calcareous particles. 



The capsule of the otolith, or rudimentary ear, is also 

 shown in Fig. 27, ot, projecting in this case from the sur- 

 face of the ganglion. 



The eye {ey') can also be readily dissected out, and the 

 parts examined. It consists of a lens, a black floor or 

 retina, and a large nerve distributed over its lower part, 

 outside of the retina. 



The nerve of the eye penetrates the retina or black layer 

 in the vertebrates, and spreads itself out on the bottom of 

 the capsule inside of this layer. This peculiarity has no 

 parallel in the structure of the Molluscan eye, with the 

 exception of the singular form known as Onchidium, This 

 animal, a naked marine species, a degenerate form of land 

 mollusc, has a number of eyes gathered in groups upon 

 the skin of the back ; and each of these, as Semper has 

 shown, has the peculiar structure of the vertebrate eye ; 



