CIRCULATION. 



649 



Fig. 320. 



Cuttle-fish. 



the gills has a dilated contractile portion (B*), 

 which dilatations may be considered as bran- 

 chial hearts, so that there are three separate 

 contractile portions of the circulatory system. 

 In the Gasteropoda and Pteropoda, there is 

 only one heart. This organ is strong and mus- 

 cular, provided with valves, and consisting of 

 an auricular and a ventricular cavity (figs. 321 

 and 322, A, H). In the Testaceous Acephala, 

 the heart is nearly of the same structure as in 

 the orders just mentioned, but less fully deve- 

 loped. In most of them, as also in the Gas- 

 teropodous Mollusca, the rectum passes through 

 the ventricle. The auricle is occasionally 

 double. The Brachiopoda have two aortic 

 hearts, but of a very simple structure, not 

 being divided into auricular and ventricular 

 portions. The naked Acephala, such as the 

 Ascidiae, have the simplest heart of all the 

 Mollusca, consisting of a thin membranous 

 ventricle apparently without valves. 



In all these animals, the course of the blood 

 is generally considered to be the following: 

 Arterial blood only passes through the systemic 

 or aortic heart (or hearts where this organ is 

 double), and is carried to the system by the 

 branches of the systemic arteries (A, a). The 



VOL. I. 



altered blood, returning in the veins of the 

 system, is collected into one or more trunks 

 ( F), and carried in the subdivided branches 

 of these (Jig. 321, Jig. 322, B) to the re- 



Fig. 321. 



Helix. 



spiratory organ, which consists of branchial 

 plates or fringes in the greater number, but in 

 some of the Gasteropoda, as in the Garden- 

 Snail, of pulmonary sacs. In most cases, the 

 whole of the blood returning from the system 

 passes through the respiratory organ. In 

 others, especially in some Bivalves, the vena 

 cava or systemic veins send branches directly 

 to the auricle as well as to the gills. 



In the compound Ascidiae, Mr. Lister* has 

 recently discovered one of the most remarkable 

 modifications of the circulation with which we 



* Philos. Trans. 1834, p. 378. 

 2 u 



