424 THE ANATOMY OF INVERTEBRATED ANIMALS. 



side, with the pedal and anterior parieto-splanchnic ganglia, 

 both of which are infra- or post-cesophageal. But in many 

 cases (most Nudibranchiata) the pedal and parieto-splanch- 

 nic ganglia are approximated to the cerebral ganglia (the 

 latter being supra-cesophageal), and are united by long sub- 

 cesophageal commissures. In others, as in most Pteropoda, 

 the pedal and parieto-splanchnic ganglia are sub-cesophageal ; 

 while the cerebral ganglia, brought close to them, are united 

 by a supra-oesophageal commissure. 



Accessory ganglia are frequently developed in the region 

 of the heart and branchiae, on the nerves of the parieto- 

 splanchnic system. 



A complicated system of visceral nerves is distributed 

 over the whole length of the alimentary canal, the genital 

 organs, and various parts of the vascular system, in many 

 OdontophoraS 



Two auditory vesicles usually exist, and very generally ap- 

 pear to be sessile upon the pedal ganglia. In the Heteropoda, 

 in many JVudibranchiata, as shown by Hancock, and in nu- 

 merous genera of Branchio- and Fulmo-g aster opoda, which 

 have been carefully examined by Lacaze-Duthiers, 2 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. 



Olfactory organs certainly exist in the Cephalopoda in 

 the form of saccular involutions of the integument near the 

 eyes ; and it is very probable that the integument of the ten- 

 tacula, or of the lips, may subserve the same function in the 

 Gasteropods. 



Eyes are generally present, and are limited to two, situ- 

 ated in the head. They resemble the vertebrate eye in struct- 

 ure, so far as they possess a concave retinal expansion, and 

 usually, in front of this, a vitreous humor, lens, and cornea. 



1 See especially Hancock and Embleton, " The Anatomy of Doris." (" Phil. 

 Trans.," 1852.) 



2 " Otocystes des Mollusques." ("Archives de Zoologie Expdrimentale," 

 1872.) In the memoir the origin of the acoustic nerves from the cerebral 

 ganglia is determined in so many Pulmo-gasteropoda {Limax, Avion, Testacella, 

 Clausilia, Zonites, Helix, Succinea, Physa, Lymnaivs, Ancylus) and Branchio- 

 gasteropoda {Neritina, Paludina, Cyclostoma, Pileopsis, Calyptrcea, Natica, 

 'JVassa, Trochus, Murex, Cassidaria, Purpura, Patella, Haliotis, Philine, Apla- 

 sia, Lamellaria), that there is a large basis for the generalization that this 

 mode of origin is universal. Moreover, according to Lacaze-Duthiers, the 

 same law holds good for the Cephalopoda. Such being the case, the question 

 suggests itself whether the connection of the nerves of the otocysts with the 

 pedal ganglia, which obtains universally among the Lamellibranchs, indicates 

 their real or only their apparent origin. 



