ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 585 



constancy in the position of the organ, Dr, Spengel is compelled to 

 believe that this group has arisen from one in which it was present on 

 both sides, and not from the Tectibranchiata, which have only one on 

 the right side. 



Similarly the " ciliated organs " detected by Gegenbaur in the 

 Pteropoda are referred to the same category. 



Speaking, for convenience, of a primitive mollusc, we find that such 

 a creature had an enteron running from before backvs^ards, a median 

 heart with two auricles, which received the efferent vessels from two 

 gills, probably from two kidneys, which opened to the right and left 

 of the anus ; and that this creature had a symmetrically arranged 

 nervous system, consisting of three pairs of peri-oesophageal ganglia, 

 connected by three connectives and three commissures ; of these the 

 visceral commissure was very long, and had intercalated in it several 

 visceral ganglia, of which the two lateral gave off nerves, which passed 

 to a ganglionated sensory (olfactory) organ placed at the base of either 

 gill. By turning this creature through 180° we get the organization 

 of the Zeugobranch Prosobranchiata ; thence by the loss of the organs 

 of the right (in the primitive mollusc left) side, we get the Aniso- 

 branch Prosobranchs (except Heteropoda). The forms which did not 

 undergo this change gave rise to the Opisthobranchs, Pulmonata, or 

 Pteropods. These considerations lead to the following scheme of 

 classification : — 



Gastropoda. 



1st Order. Streptoneura. 



1st Suborder. Zygobranchia. 

 2nd „ Azygobranchia. 

 2nd Order. Eutliyneura. 



1st Tribe. Ichnopoda. 

 2nd „ Pulmonata. 

 3rd „ Pteropoda. 



Coming next to the Lamellibranchiata, the author gives a revised 

 account of the nervous system. He thinks that it consists of two 

 cerebral (or, rather, cerebro-pleural) ganglia, and two pedal ganglia, 

 with two cerebro-pedal connectives and a visceral commissure, in 

 which there are two visceral ganglia, each of which is connected by an 

 olfactory ganglion with an epithelial olfactory organ. Save, then, 

 that there are no pleural ganglia, we here find the arrangements of 

 the typical primitive mollusc. The olfactory organs as seen in Area 

 NocB appear to be represented by a transverse band of pigment placed 

 between the hinder end of the foot and the anus ; where, as in 

 Anodonta and Unio, there is no pigment, we find a high, cylindi-ical 

 epithelium. 



The last class to be considered is that of the Cephalopoda, and here 

 the author is far less full than in the previous cases. He thinks that 

 it is possible to derive the group from his primitive mollusc ; if the 

 funnel corresponds to the foot, the whole circumanal plexus has passed 

 to the ventral surface ; if the arms correspond to the foot, then the 

 pallial cavity is dorsal in position. In either case the visceral com- 



