62 Transactions of the 



I am beginning to suspect that the nervous system of the 

 invertebrata is developed, hke that of the vertebrata, externally to 

 the serous layer of the blastodermic membrane. I expect soon 

 to be able to throw some light upon this subject ; let it suffice at 

 present to say that it is possible that the cellular layer of the 

 integument of Nematoids may represent the serous layer of the 

 blastodermic membrane, and this is apparently folded so as to 

 invest the lateral bands. The ento-thoracic growths of the integu- 

 ment in insects point to the same conclusion. But this is mere 

 theory with our present knowledge. 



From my observations I conclude that the nervous system in 

 the Scolecida is not formed on so variable a type as imperfect 

 observations have hitherto led us to believe ; but that it is entirely 

 transitional between that of the Annehda and Echinodermata. 



In the Echinodermata we find a ganglionic ring around the 

 pharynx giving off nervous cords in a radiate manner, the cords, 

 like those in Ascaris, being ganglionic. In Ascaris, if I am right, 

 we have a ganglionic ring giving off several short and two long 

 ganglionic nerve-bands. In the earth-worm, according to Claparede, 

 we have a single ventral nerve-cord connected with a ganglionic 

 ring. A step leads us to the condition seen in many Annelida — 

 a single ventral nerve-cord with ganglionic enlargements, and the 

 transition is easy to the whole series of Homogangliata. 



If we compare the nervous system of Tetrastema, one of the 

 Nemertids, with that of Ascaris, as I have described it, we shall be 

 struck with their close similarity. In Tetrastema we have the 

 lateral disposition of the main nerve-trunks, as in Ascaris ; but 

 they are no longer ganglionic bands. We have the oesophageal 

 ring, but we have a pair of cephalic ganglionic enlargements 

 added. 



From all that I have said it will be apparent that the Nematoid 

 worms stand in a clearly intermediate position between the Echino- 

 dermata and Annelida. The water vascular system with its vesicles 

 remind one strongly of an Echinoderm ; the pharynx, pharyn- 

 geal teeth, and segmented integument are clearly those of an Annelid, 

 whilst the nervous system is more nearly like that of the earth- 

 worm than that of an Echinoderm. In Gordius I think there can 

 be no doubt of this, where there is but a single ventral cord. I 

 cannot agree with Dr. Bastian's view that the Nematoid worms are 

 more nearly alhed to Echinoderms than to Scolecida, although I 

 must think much credit is due to him for having first pointed out 

 strong affinities with the Echinodermata, — not stronger affinities, 

 however, than those known to exist between Nemertids and 

 Echinoderms. 



The transition from the form seen in Ascaris to that of the 

 anenterhelminths, especially Echiuorhynchus, which has the lateral 



