82 RESPIRATION. 



they require ; the most active and energetic races live only in 

 shallow water, or near the surface ; those found in very deep 

 water are the lowest in their instincts, and are specially organized 

 for their situation. Some water- breathers require only moist sea 

 air, and a bi-diurnal visit from the tide — like the periwinkle, 

 limpet, and Kellia ; whilst many air-breathers live entirely in 

 the water or in damp places by the water-side. In fact, the 

 nature of the respiratory process is the same, whether it be 

 aquatic or aerial, and it is essential in each case that the surface 

 of the breathing organ should be preserved moist. The process 

 is more complete in proportion to the extent and minute sub- 

 division of the vessels, in which the circulating fluid is exposed 

 to the revivifying influence. 



The respiratory sj'stem is of the highest importance in the 

 economy of the MoUusca, and its modifications aff'ord most 

 valuable characters for classification, the or-ders being founded 

 upon it. 



This classification may be thus stated : 



MOLLUSCA. 



Encephala. 



(Provided with a head : shell when present, univalve.) 



Class Cephalopoda. 



Order DibrancMata^ Owen. Breathing by two gills or branchiae. 



Order Tetrabranchiata, Owen. Branchiae four. 



Class Gastropoda. 



Order Nucleohranchiata^ Blainv. Respiratory organs forming 

 a nucleus on the posterior part of the back, simple and some- 

 times not specialized. 



Order Prosobranchiata, M. Edw. Branchiae pectinated or plume- 

 like, situated in advance of the heart (Sea Mollusks with spiral 

 shells). Includes the Cuvierian orders, Pectinibranchiata, 

 Scutibranchiata, Cyclobranchiata, Tubulibranchiata. 



Order Pulmonifera^ Cuv. Breathing by lungs. (Terrestrial, or 

 aquatic.) 



Order Opisthobranchiata^ M. Edw. Branchise arborescent or 

 fasciculated, exposed on the back and sides of the body near 

 its posterior end. (Sea Slugs, Nudibranchiates). Includes the 

 Cuvierian orders, Tectibranchiata, Inferobranchiata, Nudi- 

 branchiata. 



Order Aporobranchiata, Blainv. Respiratory organ little more 

 than a ciliated surface, eithei* situated at the extremity of the 



