ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 25 



y. Pteropoda. 



Nervous System of Pteropods.* — Dr. P. Pelseneer has studied the 

 nervous system of Pteropods, in regard to which a certain degree of 

 vagueness has hitherto existed, 



(1) In Gymnosomatous Pteropods, the central nervous system, com- 

 pared with that of thecosomatous types, is characterized by the position 

 of the cerebral ganglia, which are apposed one upon the other, and 

 situated on the superior surface of the oesophagus. (2) In all genera 

 except Halopsyche the pleural ganglia are paired, and not unpaired as 

 Von Ihering Las maintained. Each pleural ganglion gives origin to a 

 nerve which anastomoses with a pedal (lateral cervical) nerve. All the 

 Gymnosomata exhibit a double pedal commissure. (3) The buccal 

 appendages of Clione and Pneumoolermon are innervated by the cerebral 

 ganglia, and not by the pedals as Gegenbaur stated. These appendages 

 are therefore not pedal in their nature. (4) The visceral commissure 

 of typical Gymnosomata exhibits two superposed ganglionic masses, 

 which give origin to the asymmetrical nerves, three from the left, and 

 one from the right, and not to symmetrical branches as most authorities 

 describe them. 



As to Thecosomatous Pteropods, the central nervous system has been 

 often described. Pelseneer contents himself for the most part with 

 emphasizing that the system is characterized (1) by the separation of the 

 cerebral ganglia, which are situated on the sides of the oesophagus, and 

 united by a long supra-oesophageal commissure, (2) by the absence of 

 pleural ganglia, the pedals and viscerals being directly apposed to the 

 cerebrals from which they are separated only by a constriction, and 

 (3) by the coalescence of all the ganglionic elements of the visceral 

 commissure in a single elongated mass. The nerves which spring from 

 the visceral ganglion are in origin asymmetrical. The left portion of the 

 ganglion gives origin to three principal nerves, the left pallial and two 

 viscerals, while the right portion only gives rise to the right pallial. 

 Souleyet alone has given a correct representation of this fact. The 

 nervous system of Cymbulia is discussed in detail. Halopsyehe among 

 Gymnosomata agrees with Cymbulia. Three types may be distinguished : 

 one represented by the two genera just named, a typical Gymnosomatous, 

 and a typical Thecosomatous arrangement. 



The author then discusses the homologies between the various 

 Pteropod types, and between these and molluscs generally. (1) The 

 two lateral ganglia — right and left — of Malopsyche and Cymbulia are 

 homologous with the anterior or pallial visceral ganglia of other 

 molluscs, for they give origin to nerves which supply similar regions. 

 The unpaired median ganglion of the same genera corresponds to the 

 united posterior visceral ganglia of other molluscs, for they give rise to 

 nerves which supply the circulatory, respiratory, and reproductive 

 apparatus. (2) The left ganglion of typical Gymnosomatous Pteropods 

 is homologous with the left anterior visceral, and posterior visceral 

 together, while the right ganglion of the former corresponds to the right 

 anterior visceral. (3) The left portion of the visceral ganglion of 

 typical Thecosomata is homologous with the left anterior visceral 

 and posterior visceral together, while the right half corresponds to 

 the anterior right visceral. The visceral ganglionic mass of typical 



* Arch, do Biol., vii. (1887) pp. 93-120 (1 pi.). 



