Respiration in hwertehrate Animals, [d B2^: 



determined that in the scale stretching between the Patellidae and 

 Cephalopods, the nervous and circulatory systems display few- 

 evidences of advancement. 



In the lowest Gasteropod the heart is as perfect in structure 

 as in the highest Cephalopod. When the branchial organ is 

 symmetrically developed, the heart has two auricles. This is the 

 case in Chiton, Fissurellay Emarginula, Haliotis, Tethys and 

 Janus, and less completely in the Eolidse. In all other Gaste- 

 ropods the auricle is single. The position of the heart depends 

 upon that of the respiratory organs. It is situated on the right 

 side of the back in the Pulmonata, most Tectibranchiata, and 

 the dextral Pectinibranchiata, and in all the Limacidse ; it is on the 

 opposite side in the sinistral Gasteropods, Ancylus and Haliotis ; 

 it is to the left of the dorsal median line in Carinaria, Clio, 

 Hyalcea and Cleodora ; and near the hinder end of the body in 

 Firola and Atlanta. In Dentalium, Tritonia, Scyllcsa, Phyllidea^ 

 it is on the dorsal median line. The heart is furnished with a 

 distinct pericardium in all Gasteropods, save the Apneusta, 

 where it is not clearly defined. In all those genera whose 

 branchial organs are symmetrical, the ventricle and aorta are 

 directed forwards, but in the turbinated genera they are directed 

 backwards. 



Between the heart and respiratory organs in this class twor 

 relationships are discernible. In the first the heart is placed be- 

 tween the head and the branchise — Prosobranchiata (M.-Edw.) ; 

 in the latter, between the tail and the branchise — Opistho- 

 branchiata. 



5^ In all, there is between the branchise and the heart a most 

 intimate juxtaposition. In all families the heart is systemic. In 

 no single species is it pulmonic or branchial. In all, the auricle 

 or auricles receive the blood immediately from the respiratory 

 organ. The heart in the testaceous Gasteropods, spiral and 

 otherwise, is always placed at the posterior end of the branchial 

 cavity, or in other words, is fixed at that extremity of the 

 hv2i\\Q\n.^ farthest from the entry of the aerating fluid. In Den- 

 talium this rule is not broken, because here the water enter&^t 

 the posterior instead of at the anterior orifice of the mantle, bna 



The same general observations apply to the nervous system. 

 Souleyet first explained that the parts which by their constancy 

 and ftxity constitute the essential centre of this system in the 

 Mollusca, are always grouped around the oesophagus. The 

 others should only be regarded as difi'erenfc degrees of develop- 

 ment of these central portions, and this is proved by their degra- 

 dation or disappearance in proportion as we descend in animals 



22* 



