Respiration in Invertebrate Animals. 323 



determined that in the scale stretching between the Patellidse 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, Fissurella, Emurginula, 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 Limacidae ; 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, Scyllaa, Phyllidea, 

 it is on the dorsal median line. The heart is furnished vrith 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 two 

 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 branchiae — Opistho- 

 branchiata. 



In all, there is between the branchiae 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 })osterior end of the branchial 

 ca\aty, or in other words, is fixed at that extremity of the 

 branchiae /crMe^^ from the entry oi the aerating fluid. In Den- 

 talium this rule is not broken, because here the water enters at 

 the posterior instead of at the anterior orifice of the mantle. 



The same general observations apply to the nervous system. 

 Souleyet first explained that the parts which by their constancy 

 and fixity constitute the essential centre of this system in the 

 MoUusca, are always gi-ouped around the oesophagus. The 

 others should only be regarded as diff"erent 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* 



