VASCULAR SYSTEM. 37 



have so multiplied observations upon this point, that their exis- 

 tance in all prosobranchiates may be considered highly probable. 



Two auditory vesicles usually exist, and very generally appear 

 to be sessile upon the pedal ganglia, where they appear as small 

 white points. In the heteropoda, in many nudibranchiata, as 

 shown by Hancock, and in numerous genera of branchio- and 

 pulmo-gasteropoda, which have been carefully examined by 

 Lacaze-Duthiers, however, there seems to be no doubt that the 

 auditory nerves arise from the cerebral ganglia, even though the 

 vesicles may be situated close to the pedal ganglia.* 



Within the vesicles are found, in many univalves a single large 

 otolith, whilst in others numerous smaller ones exist, or even, as 

 in the oriental Melania and Melanopsis, a large laminated otolith 

 together with small crystalline ones, f The auditory nerve 

 divides upon the vesicle into a number of branches ; and the 

 vesicles are probably connected with the external world by means 

 of an auditor} 7 canal ; at least such a canal exists (according to 

 Ad. Schmidt) in Helix, and Iv Hiker discovered it in the cepha- 

 lopods. 



Fa.sc u la r System. 



The circulation in mollusca varies greatly as to complexity, 

 according to the higher or lower organization of the animals. 

 In the prosobranchiates (plate 6) the circulation is the most 

 complicated, and yet comparatively to vertebrates, simple. Ar- 

 teries proceed from the heart to the various organs, where they 

 subdivide and terminate in line capillary vessels. There are no 

 venous capillaries, and the blood flows freely around the organs 

 in the body. There are valves in the aorta and auricles of the 

 heart which permit the flow of blood only from the auricle to the 

 aorta. From the body cavity the blood flows into veins, some 

 of which conduct to the branchiae whilst others pass directly to 

 the ventricle of the heart. In some genera the arteries do not 

 all end in capillary vessels, but in a portion of them, and espe- 

 cially in the anterior part of the body these are replaced by 



* Huxley, Anat. Invert., 494. 



f Hermann von Jhering records the existence of otoliths in Cassis, 

 Cassidaria, Conus, Buccinum, Nassa, Murex brandaris, Fusus Syracusanus, 

 Columbella rustica and Mitra ebenns. Sitzungsb. Phys. Med. So-. Erlan- 

 gen, ix, 63, 1877. 



