[130] The Hertwigs hold that there is a distinct part of the nervous system which was at first differentiated in the mesoblast in many types, amongst others the Mollusca. The evidence in favour of this view is extremely scanty and the view itself appears to me highly improbable.
[131] The reader is referred for this subject to the valuable memoirs which have been recently published by the Hertwigs, especially to No. [270]. He will find a general account of the subject written before the appearance of the Hertwigs’ memoir in pp. 180-182 of Volume II. of this treatise.
[132] It would be interesting to know the history of the various nervous structures found in the walls of the alimentary tract in the higher forms. I have shewn (Development of Elasmobranch Fishes, p. 172) that the central part of the sympathetic system is derived from the epiblast. It would however be well to work over the development of Auerbach’s plexus.
[133] The wide occurrence of this process was first pointed out by Rabl. He holds, however, a peculiar modification of the gastræa theory, for which I must refer the reader to his paper (No. [284]); according to this theory the mesoblast has sprung from a zone of cells of the blastosphere, at the junction between the cells which will be invaginated and the epiblast cells. In the bilateral blastosphere, from which he holds that all the higher forms (Bilateralia) have originated, these cells had a bilateral arrangement, and thus the bilateral origin of the mesoblast is explained. The origin of the mesoblast from the lips of the blastopore is explained by the position of its mother-cells in the blastosphere. It need scarcely be said that the views already put forward as to the probable mode of origin of the mesoblast, founded on the analogy of the Cœlenterata, are quite incompatible with Rabl’s theories.
[134] Zoologischer Anzeiger, No. [52], p. 140. This form has been named by Kowalevsky Cœloplana Metschnikowii. Kowalevsky’s description appears, however, to be quite compatible with the view that this form is a creeping Ctenophor, in no way related to the Turbellarians.
[135] For numerous instances of this kind, vide Chapter XI. of Vol. III.
[136] It has long been known that land and freshwater forms develop without a metamorphosis much more frequently than marine forms. This is probably to be explained by the fact that there is not the same possibility of a land or freshwater species extending itself over a wide area by the agency of free larvæ, and there is, therefore, much less advantage in the existence of such larvæ; while the fact of such larvæ being more liable to be preyed upon than eggs, which are either concealed, or carried about by the parent, might render a larval stage absolutely disadvantageous.
[137] The phosphorescence of many larvæ is very peculiar. I should have anticipated that phosphorescence would have rendered them much more liable to be captured by the forms which feed upon them; and it is difficult to see of what advantage it can be to them.
[138] The larva of the Brachiopoda does not possess most of the characters mentioned below. It is probably, all the same, a highly differentiated larval form belonging to this group.
[139] There is some uncertainty as to the development of the œsophagus in the Echinodermata, but recent researches appear to indicate that it is developed from the hypoblast.