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



first formulated by Gegenbaur, about the phylogenetic origin of the ventral nerve-cord 

 and 03sophageal ring of the Annelida out of ancestors with lateral cords, has obtained new- 

 support from the arrangement which was met with in the species just mentioned. And 

 just as we have before tentatively discussed the question, in how far remnants of the 

 lateral cords were retained in those descendants in which the median one had been raised 

 to the dignity of a medulla spinalis (the Vertebrata), we might now consider whether any 

 remnants of the median dorsal cord are retained in those descendants in which the lateral 

 cords have differentiated into brain-lobes, oesophageal ring, and ventral cord (the 

 Annelida). To this question I have no definite answer to offer, but I may call attention 

 to the significant fact that the beautiful and exemplary investigations into the embryonic 

 development of Lopadorhynchus, very recently published by Kleinenberg,^ have 

 demonstrated the existence in the larva of that Annelid, of a nerve-stem answering to the 

 conditions here required. It is dorsomedially situated, it is anteriorly connected with 

 the brain, or rather with a transverse nerve-tract (Kleineuberg's prototrochal nerve-ring), 

 which in its turn is connected with the brain,^ it ajjpears to be connected close to the 

 anus with the ventral cord (the fused lateral stems), and though appearing in early 

 larval life, and having only a temporary existence, it is regarded by Kleinenberg as 

 having considerable physiological importance. If the light in which I am inclined to 

 look at it is not deceptive, its morphological significance also can hardly be overrated. 



In closing this chapter of general considerations, we may once more bring before 

 our minds the proposition with which it was opened. We have here and in the foregoing 

 chapters adduced facts and arguments which appear to speak in its favour ; we will once 

 more rapidly envimerate the common characteristics of Nemertea and Coelenterata, as 

 well as those of Nemertea and Chordata. 



The Coelenterate characteristics that are also found in the Nemertea are the fol- 

 lowing : — 



a. The presence of nematocysts in the proboscidian epithelium. 



h. The elaborate nerve-plexus in the integument, and its histological features. 



c. The presence of epiblastic muscle-fibres separate from the general body- 



musculature. 



d. The presence and the chemical constitution of a sometimes very massive 



intermuscular jelly, by which the other internal organs are at the same 

 time surrounded. 



e. The mode of development of the mesoblast (at least in Lineus ohscurus), 



which is less specialised than in most other Invertebrates. 



f. The absence of any distinct enterocoele. 



1 Zdtschr.f. vnss. ZooL, Bd. xliv.. Heft, i, ii., October 1886, p. U)7 ; pi. vii. fig. 27a. 



2 For comparison with the Nemertea, cf. pi. xvi. fig. 1. 



