XERVOUS SYSTEM. 341 



segmented strands, although in a tew cases (some Crustacea and 

 Gephyrea) these cords, by an abbreviation in development, arise as an 

 unpaired median thickening of the epiblast. 



The form of nervous system of the Chaetopoda, Arthropoda, and 

 Gephyrea is clearly therefore to be derived, as was first pointed out 

 by Gegenbaur, from a more or less similar type to that now found in 

 the Nemertines ; and as suggested in the chapter on larval forms 

 (vide p. 312) may perhaps be derived from the elongation of a cir- 

 cular ring, of which the anterior end has become developed into 

 the supracesophageal ganglia, the lateral parts into the two lateral 

 strands, while the posterior part persists in some forms in the junction 

 of the ventral cords above the anus (Enopla and Peripatus). 



MolhlSCa. 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 011 the subject, is somewhat ab- 

 normal '. 



In the Gasteropoda and Pteropoda the supraoesophageal 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 L max). 

 In either case the supraoesophageal 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 Chsetopods 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 Araneiua." Quart. J. of 

 Micr. Science, Vol. xx. 1880. 



(328) B. Hatscbek. "Beitr. z. Entwicklung d. Lepidopteren." Jenaische Zeit- 

 Kchrift, Vol. xi. 1877. 



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

 zoicles.'' Quart. J. of Micr. Science, Vol. xix. 1879. 



1 Vide Vol. i., pp. 22(i, -227. 



2 " Ueber Entwicklungsgeschichte von Teredo." Arbeit, n. d. zool. lust it. Wien, 

 Vol. in. 1830. 



