262 Transactions of the Royal Microscopical Society. 
layer of the embryo. I throw this out, however, as a mere sugges- 
tion. 
The next type of nervous system to which I would draw your 
attention is common amongst the Scolecida, especially those which, 
like the Nematoids and Gordiaceas, hear many resemblances to the 
Echinodermata. The nervous system consists of a ring of nerve 
tissue around the pharynx, histologically like that of an Echinoderm, 
and closely related to the cellular layer of the integument, of which I 
believe it to be a differentiation. From this two or more bands of 
similar structure pass, according to my own observations,* to the anal 
pole of the body. This seems to be the case in the Ascaris at least, 
where I have traced short rudimentary intermediate bands passing 
back from the oral ring between the main nerve bands; these 
apparently represent the three missing radiating nerves of the 
typical Echinoderm. Some forms, as Mermis, have three instead of 
two well-developed nerves passing back from the oesophageal ring. 
In the earth-worm, according to the elaborate researches of 
E. Claparede (whose much- lamented death is one of the greatest 
losses zoology has met for years), the structure of the nervous system 
follows a precisely similar type. In this case, however, only two 
bands pass back from the oesophageal nerve ring, and these are 
closely united and lie in the middle line on the ventral aspect of the 
body. Histologically, their structure is nearly like that of the 
Echinodermata, but the nerve cells show a tendency to become 
aggregated in places ; this is the first indication of the formation of 
distinct ganglia. 
The transition from this condition to that observed in the higher 
Annelida, Insecta, and Crustacea, is not difficult. The nerve cells 
become collected in separate groups, or ganglia, and the remaining 
portions of the cords form the longitudinal commissures between 
them. The nervous system also becomes more central by the fold- 
ing inward of the serous layer of the blastoderm to form the 
internal skin growths already mentioned. Thus we arrive at the 
highest forms, and we trace connecting links between the whole 
great series of organisms grouped under the sub-kingdoms Annuloida 
and Annulosa. 
Before concluding, I cannot help referring to the visceral system 
of nerves so well marked in many Articulates, which is clearly 
developed from the mesoblast, as the sympathetic system is in the 
Vertebrata. I would draw attention to the great similarity of the 
complex visceral plexuses and the chain of visceral ganglia in the 
two great sub-kingdoms. These latter are undoubtedly differently 
placed, but their development from the mesoblast is a strong indica- 
tion that they are to some extent homologous as well as analogous 
systems. 
* ‘ Monthly Microscopical Journal,’ January, 1871. 
