ORGANS OF THE SENSES. 291 



'animals have distinct organs to enable them to be sensibly 

 affected by odorous effluvia, but the air-breathing annelides 

 among the articulata have been supposed to perceive them 

 by the parietes of their mouth or by the lateral pores of 

 their air- sacs. The sense of smell so distinctly manifested 

 and so delicate in insects, has also been referred to the 

 same parts of their body, or to their palpi, or to their 

 cesophageal sacs, or to the delicate subdivided laminated 

 extremities of their long flexible antennae, and the inner 

 pair of these organs in the Crustacea have been considered 

 as the seat of the same sense. The labial appendices of the 

 conchifera and other molluscous classes, the entrance of 

 the respiratory sacs in pulmonated gasteropods, the highly 

 sensitive tentacula covered with a delicate mucous mem- 

 brane, and even the whole surface of the skin in the higher 

 mbllusca, have been regarded as the organs through which 

 these animals receive impressions of odorous emanations. 



The organs of smell, on which the olfactory nerves are 

 entirely distributed, are very obvious in fishes, although 

 they do not communicate with the respiratory organs or 

 with the cavity of the mouth. They are laminated organs 

 placed in cavities or depressions excavated on the anterior 

 part of the face and protected by cartilages as in higher 

 animals, but have yet no posterior opening into the interior 

 of the body on account of the density of the element here 

 respired. The olfactory nerves, arising alone from the 

 rudimentary hemispheres of the brain and provided with 

 large olfactory tubercles, perforate the anterior part of the 

 skull corresponding with the ceribriform plate of the ethmoid 

 bone, and immediately spread upon the numerous parallel 

 or radiating laminae covered with a delicate and extensive 

 mucous membrane. These numerous nasal laminae covered 

 with the petuitary membrane are thus more or less exposed 

 on the surface of the face to the contact of the surrounding 

 element, and a fold of the integuments supported by a 

 cartilaginous plate generally hangs like a valve over the 

 middle of each nasal cavity. The nasal cartilage protecting 

 each cavity partially divides its entrance into two, so that 

 the water passes freely through its interior and over the 

 extensive' surface of the olfactory laminae during the lateral 

 motions of the head and the progressive movements of the 



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