1898.] voyage to Melanesia during the years 1894 — 1897. 399 



casteaux Group, to the east of New Guinea. While the animals 

 of N. pompilius and N. macromphalus are almost, if not quite, 

 indistinguishable from one another, that of N. umbilicatus differs 

 markedly from both, especially in the surface texture of the hood, 

 which is coarsely areolated, the polygonal block-like areas being 

 separated by deep valleys. 



5. In any attempt to interpret the morphology of the tenta- 

 cular appendages of Nautilus and the arms of the Dibranchiate 

 Cephalopods, if function, innervation, and development are re- 

 jected as evidence of their pedal origin, the only way left open 

 to approach the subject seems to be by taking into consideration 

 the general phenomena involved in cephalogenesis. In all other 

 animals which possess a compound head, the latter is not pro- 

 duced by elaboration of the pre-existing simple head with its 

 praeoral lobe, but by the incorporation of other parts of the body 

 into the cephalic complex. Thus while in Arthropods we have 

 the cephalothorax, in the Cephalopod Molluscs we have, viewed 

 from the standpoint here adopted, the cephalopodium. 



II. Gtenoplana. This remarkable animal appears to be the 

 existing representative of the type from which the Ctenophora 

 and the Plathelminthes diverged ; the former being its pelagic 

 and the latter its littoral descendants. The importance of Gteno- 

 plana lies in the fact that it presents a transition from biradial 

 to bilateral symmetry, the transitional feature being its dorso- 

 ventrality. 



Some authorities consider Ctenoplana only as a slightly modified 

 Ctenophore. From this point of view, however, Ctenoplana loses 

 much of its interest and all of its importance; but I do not think 

 that there is sufficient justification for such a depreciatory attitude. 

 It may indeed be said that Ctenoplana is no more a Ctenophore 

 on the one hand or a Planarian on the other than Peripatus is a 

 Chaetopod or a Myriapod. For further details on Ctenoplana I 

 may refer the reader to my paper in the Quart. Journ. Micro. 8c. 

 Vol. xxxix., p. 323. 



III. Enter opneusta. In Spengel's great Monograph of this 

 group there are two theoretical conclusions to which I wish to 

 refer. The first is that the Enteropneusta have no Chordate 

 affinities ; and the second, that the genus Balanoglossus (sensu 

 stricto) is the most primitive type of the group. As this genus 

 is the one which is least capable of being compared, in any 

 detail, with Amphioxus, the latter conclusion, if true, would tend 

 to confirm the former. 



In the section Chlamydoihorax of the genus Ptychodera the 

 gill-slits open freely to the exterior throughout the greater part of 



33—2 



