412 THE SENSORIAL FUNCTIONS. 



animals are perfectly aquatic ; breathing water by 

 means of gills, and having all their organs formed 

 on the model of the fish. Their nasal cavities are 

 not employed for respiration at this early period; 

 nor even for some time after they have begun to 

 take in air, which they do by the mouth, swal- 

 lowing it in small portions at a time, and after- 

 wards throwing it out in bubbles by the same 

 orifice. But when they quit the water, and 

 become land animals with pulmonary respiration, 

 the nostrils are the channels through which the 

 air is received and expelled ; and it is here also 

 that the sense of smell continues to be exercised. 

 We know very little respecting the seat of 

 the sense of smell in any of the invertebrated 

 animals, though it is very evident that insects, 

 in particular, enjoy this faculty in a very high 

 degree. Analogy would suggest the spiracles as 

 the most probable seat of this sense, being the 

 entrances to the respiratory passages. This 

 office has, however, been assigned by many to 

 the antennae ; while other entomologists have 

 supposed that the palpi are the real organs of 

 smell.* Experiments on this subject are at- 

 tended with great difficulty, and their results 

 must generally be vague and inconclusive. 

 Those which Mr. P. Huber made on bees seem, 

 however, to establish, with tolerable certainty, 

 that the spiracles are insensible to strong odours, 



* On the subject of this sense in insects, See Kirby and 

 Spence's Introduction to Entomology, vol. iv. p. 249. 



