RESPIRATION. 83 



body and unprotected b}' a mantle, or included in a branchial 

 chamber with an opening in front. 



Acephala. 



Class Pelecypoda. 



Order LamelUbranchiota, Bl. Branchiae or gills lamelliform, 

 specialized from a portion of the mantle. 



MOLLUSCOIDA. 



Class Brachiopoda. 



Order PalliohraruMata^ Bl. Breathing by the arms, assisted 

 by the mantle. 



It will be seen that respiration b}^ means of hranchise or gills 

 is the rule with mollusks, those respiring by lungs being con- 

 fined to a portion only of a single one of the five classes, and not 

 including marine species. A few appear devoid of special respir- 

 atory organs, that office being performed by the general envelope 

 of the body. 



Cephalopoda. The gills form a cylinder in Octopus and Sepia, 

 and in Loligo and other genera they are in the form of a half- 

 cylinder ; they are two in number in the naked cephalopods, 

 (vi, 80, 82), as well as those possessing an internal shell ; and 

 four, arranged a pair on each side, in the Nautilus ; hence the 

 terms dibranchiata and tetrabranchiata, forming the highest 

 divisions of the class Cephalopoda. The water finds access to 

 the gills through the large opening between the free anterior 

 ventral margin of the mantle and the body, and it is expelled 

 from the funnel bj' a muscular contraction of the wall of the 

 mantle ; assisted in some genera by a buttoning arrangement 

 described under the head of Cartilages. 



The Octopus in repose respires from twenty to thirty-eight 

 times per minute ; in movement, fifty times. The respiration of 

 Sepia is, according to Dr. Fischer's observations made at Arca- 

 chon, much more active, he counted seventy to sevent^^-two 

 respirations per minute in the adult, one hundred and forty 

 in young individuals, an inch in length. 



Gastropoda (vii, 90, 91). The branchiae in prosobranchiates 

 are small, leaf-like, hollow prolongations of the mantle, placed in 

 rows behind each other, and are usually contained in a pouch on 

 the dorsal side of the animal, forming the respiratory cavity. 

 In some opisthobranchiates the gills are lodged at the sides of the 

 body between the narrow, collar-like mantle and the broad foot ; 

 such is the position of the filamentous branchiae of Patella and 

 the laminated gills of Chiton. In the spiral species the right 

 branchia only is well developed, that on the left side being small 



