34 LAND AND FRESH-WATER SHELLS. 



Chiton this division of the auricle also obtains. In Zonites algirus, 

 according to Nalepa, and in Arion ater, according to Jourdain, 

 the transition vessels open into the venous sinuses by infundi- 

 bular orifices. In Zonites algirus Nalepa has demonstrated a 

 network of nerve-fibres in the auricle, ventricle, aorta, and larger 

 vessels, but ganglia are rare. Haller has demonstrated ganglia 

 in the cardiac walls of the marine snails, Trochus, Haliotis, and 

 Fissurella. The arteries and arterioles in Zonites algirus are 

 lined by an endothelium. "The venous system is represented in 

 part by the coelome (body-cavity), in part by vessel-like spaces, 

 the walls of which are formed of homogeneous connective tissue 

 with scattered nuclei, but which are not lined by epithelium." In 

 Chiton, Nerita, Neriti?ia, Turbo, Emarginula, Fissurella, and in 

 Parmaphorus the ventricle is pierced by the intestine.* In 

 /anus and Emarginula, besides the genera already mentioned, 

 there are two auricles. In many Prosobranchs the heart is 

 connected to the pericardium by fibrils. In Aplysia the com- 

 mencement of the aorta is strengthened by muscle-fibres, a 

 condition of things which, as Professor Owen says, resembles the 

 bulbus arteriosus of fishes. In some forms, as Pontolimax and 

 Phyllirhoe, there are no vessels except the aorta ; in Rhodope and 

 Entoconcha there are no specialised vessels at all. In Patella the 

 " transition vessels " are separate from the ccelome. In Haliotis, 

 Fissurella, and Emarginula the ventricle is double. Vulpian has 

 found that nicotine, curare, and digitalis inhibit the heart's beat 

 in about five or six minutes, just as they do in the higher 

 Vertebrates. According to Ashford the pulse of Helix hortcnsis 

 and Hyalina cellaria beats 12 to 14 per minute at 42 to 44 Fahr., 

 9 to ii at 37 to 38 Fahr., and 4 to 8 at 26 to 33 Fahr. 



THE RESPIRATORY SYSTEM. In the Pulmogastropoda breathing 

 is carried on through the pulmonary aperture which leads into the 

 mantle-sac formed by an expansion of the body-wall (mantle}. 

 The walls of the mantle are very vascular, the vessels running in 

 it being those already mentioned, the afferent and efferent pul- 



* Landsberg is opposed to the statement generally made that the intestine 

 perforates the heart in Ncritina Jluviatilis and Turbo rugosus. In the former, 

 according to his researches, the intestine passes between the heart and the 

 nephridium ; in the latter it pierces the pericardium only. 



