CIRCULATORY AND RESPIRATORY SYSTEMS. 57 



Brachiopoda. Hearts two, one on either side the ali- 

 mentary canal. Respiration by two long spiral branchiae. 



Tunicata. In Salpa, two vascular trunks are placed on 

 either side of mantle, upon the inner side of which extend 

 the branchiae. With Ascidians, a heart of a single cavity is 

 placed at lower part of body. The circulation of blood is to 

 and fro. Branchial chamber at the same time a pharynx. 



Lamellibranchiata. Heart is simple with some, but may 

 contain both auricle and ventricle. Occasionally, as in 

 Area, a distinct heart is placed upon either side of the body. 

 It is generally pierced by rectum, Ostrea (oyster) an excep- 

 tion. There are two pair of branchiae, which as four lamellae, 

 embrace either side of the abdomen and foot. Each lamella 

 is formed by a widely-projecting cutaneous fold, the two 

 leaves of which are connected by numerous transverse septa, 

 to which correspond externally as many furrows, passing 

 from the base of each brauchia to its borders. 



Gasteropoda. Heart composed of two chambers, almost 

 always in close relation with the respiratory apparatus, and 

 often pierced by rectum. Respiration mostly by branchiae, 

 occasionally by lung-like structure. The branchial appa- 

 ratus varies in its form, and may be situated upon different 

 portions of the body. Thus, the gills may be exposed in 

 tufts upon the back, as in Doris; or arranged on either side 

 of the body, in a furrow between the mantle and the foot, 

 transversely, in a single row, as in Phyllidia; or composed of 

 divided lamellae resembling divided leaves, as in Aplysia (sea- 

 hare); or foliaceous, or pyramidal, arranged in rows on either 

 side under the margin of mantle, as in Chiton; or composed 

 of numerous leavelets, arranged parallel, like the teeth of a 

 comb, as in Paludina (fresh-water snail). The pneumonic 

 apparatus is situated upon the right side of body, opening 

 externally by an aperture through mantle, and furnished 

 with a network of vessels running on its walls. The shell 

 is a special protector to the respiratory apparatus, and is 

 placed upon it even when rudimentary. 



Cephalopoda. Heart central, of two cavities, and sur- 

 rounded by pericardium. Those forms having two pairs 



