74 THE VOYAGE OF H.M.S. CHALLENGER. 



Hertwigs' objections, and maintains that these naturalists have furnished arguments in 

 favour of his hypothesis rather than of their own interpretation (loc. cit., p. 205). Kleinen- 

 berg holds that the naked nerve-cells of Hydra, that are in mutual and direct communica- 

 tion, may transmit a stimulus by contact without the intervention of a delicate network 

 of inter-cellular protoplasmic threads forming a network. He, moreover, holds that the 

 epithelial cells had all of them the double significance of nerve-cells and muscle-cells, i.e., 

 were true neuro-muscular cells before further division of labour set in, whereas the 

 Hertwigs maintain that this division of labour took place between epithelial cells that were 

 not yet physiologically so far differentiated. 



A nerve plexus, which covers a very large surface, was actually demonstrated by 

 the brothers Hertwig not only in Actinia and other Ccelenterata but also in the 

 Chsetognatha. Of the latter 0. Hertwig says : T — 



" By the fact of the nerve-fibres crossing and decussating in the most complex and 

 diverse ways, there is formed a nerve plexus which spreads over the whole surface of the 

 body, and in which the above described nerve-stems represent the single collecting tracts." 



A more or less similar plexiform arrangement of nerve-tissue has since been demon- 

 strated in nearly all the lower groups of invertebrates, Annelids 2 and Arthropods 

 excepted. Thus in the works of Loven, Greeff, Teuscher, Ludwig, and Carpenter the 

 nervous system of the Echinoderms is described as offering many analogies with the type 

 propounded as the most primitive by the Hertwigs. 



Nemertea, Turbellaria, Trematodes, and Cestodes can now be very fully compared, as far 

 as their nervous system is concerned, with Hertwig's starting point, when we consider the 

 results obtained by myself (IX, X) — which were afterwards confirmed (II) by Dewoletzky 

 — for Nemertea; by Lang, Graff, and Pintner for Turbellaria, Trematodes, and Cestodes. 



Among aberrant forms one of the most striking examples of a thick epiblastic nerve- 

 plexus with longitudinal collecting tracts is offered by Balanoglossus, as described by 

 Spengel and more lately by Bateson. We shall have occasion again to refer to this 

 interesting nervous system further on. 



For Mollusca, remnants of a more or less plexiform arrangement were found to exist 

 in the Amphineura by myself 3 (Proneomenia) and by Haller 4 (Chiton), and also in other 

 groups of Mollusca by Semper, 5 Simroth, 6 and others. 



1 Die Chaetognathen, p. 34. 



2 Lately Fraipont (Archives de Biologie, 1884, p. 274) has demonstrated the presence of an intermuscular nervous 

 plexus in Polygordius, Protodrilus, and Saccocirrus, and thus opened the possibility of also bringing the Annelids within 

 the region of comparison so far as this point of their organisation goes. Bergh describes a nerve-plexus in the larval 

 Aulostoma (Arbeit. Zool. Zoot. Inst. Wurzburg, Bd. vii. p. 238). As to Arthropods there are facts which also point in the 

 same direction, e.g., that Hoek mentions " a continuous network of ganglia and nerves " on the inner surface of the in- 

 tegument in Pycnogonida (Zool. Chall. Exp., pt. x. p. 11G). 



3 Niedcrland. Archivf. Zool., Suppl. Band, 1881. 4 Zool. Anzeiger, No. 76. 



6 Archivf. Mikr. Anat, Bd. xix., p. 124, 1877; Arbeit. Zool. Zoot. Inst. Wurzburg, Bd. iii., 1877. 

 6 Zeitschr. f. iviss. Zool., Bd. xxxii. p. 304. 



