414 MOLLUSCA. 



Mollusca. While study of the anatomy of the nervous system of the 

 Mollusca, especially of certain primitive genera (Chiton, Haliotis, Fissurella, 

 &c.) leaves little doubt that it is formed on the same type as that of the 

 groups just spoken of, the development, so far as our imperfect knowledge 

 enables us to make definite statements on the subject, is somewhat ab- 

 normal 1 . 



In the Gasteropoda and Pteropoda the supracesophageal ganglia appear 

 most probably to be developed either as paired thickenings of the epiblast 

 of the velar area, or as invaginated pits of the velar area, which become 

 detached from the surface, and then become solid (Hyaleacea and Limax). 

 In either case the supracesophageal ganglia appear to be developed quite 

 independently of the pedal ganglia. The latter, as might be anticipated, are 

 earlier in their development and more constant than the various visceral 

 ganglia ; and, if the views above expressed are correct, are homologous 

 with the ventral cord of the Chaetopods and Arthropods. Their actual 

 development is very imperfectly known. 



The most precise statements on the subject, viz. those of Bobretzky and 

 Fol, would lead us to suppose that they arise in the mesoblast, but it seems 

 more probable that they are formed as thickenings of the sides of the foot. 



In the Cephalopods all the ganglia are stated to be differentiated in the 

 mesoblast (Lankester, Bobretzky). 



Hatschek 2 has recently given a detailed description of the development 

 of the supracesophageal and pedal ganglia of Teredo. He finds that the 

 former ganglia arise as an unpaired thickening of the epiblast in the centre 

 of the velar area, and the latter as an unpaired thickening of the epiblast 

 of the ventral side of the body between the mouth and the anus. The two 

 ganglia would thus seem to be disconnected with each other in their 

 development. 



(327) F. M. Balfour. "Notes on the development of the Araneina." Quart. 

 J. of Micr. Science, Vol. XX. 1880. 



(328) B. Hatschek. " Beitr. z. Entwicklung d. Lepidopteren." Jenaische 

 Zeitschrift, Vol. xi. 1877. 



(329) N. Kleinenberg. "The development of the Earthworm, Lumbricus 

 Trapezoides." Quart. J. of Micr. Science, Vol. XIX. 1879. 



(330) A. Kowalevsky. " Embryologische Studien an Wiirmem u. Arthro- 

 poden." Mem. Acad. Petersbourg, Series vin., Vol. XVI. 1871. 



(331) H. Reichenbach. " Die Embryonalanlage u. erste Entwick. d. Fluss- 

 krebses." Zeit.f. wiss. Zool., Vol. xxix. 1877. 



1 Vide Vol. ii., pp. 273, 274. 



2 " Ueber Entwicklungsgeschichte von Teredo." Arbeit, a. d. zool. Instit, IVieit, 

 Vol. in. 1880. 



