296 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



among its adherents Leydig, Semper, Dohra, Eisig and many other distinguished 

 names. 



Without presuming to characterise the present contribution as a culmination of 

 any kind whatever it may nevertheless be said, with truth, that it falls into line 

 with the work of Johannes Miiller, Kowalevsky, Hatschek, Huxley, Lankester and others. 



I. Theory of Gill-slits 1 . 



In submitting the theory of gill-slits at which I have arrived during the course 

 of my work on the Enteropneusta I definitely assume, at the outset, that whatever 

 be the true approximate explanation of gill-clefts, it must, at all events, be sought 

 for in free-living animals possessing a straight alimentary canal and not in sedentary 

 forms, nor in purely pelagic forms, which possess a U-shap^d alimentary canal. The 

 theory suggested by Harmer based upon the anatomy of Cephalodiscus and by Brooks on 

 the basis of Appendicularia, which has recently been further elaborated by Masterman'-, 

 does not, in my opinion, account satisfactorily for the primordial origin of gill-slits, but 

 it probably does explain the retention of a single pair of gill-clefts in the above-named 

 animals. This is a point of great interest and some importance, because, the Ptero- 

 branchia probably bear the same sort of relationship to the Enteropneusta as that in 

 which the Urochorda stand to the Cephalochorda. 



In the Enteropneusta, as in Amphioxus, we observe the very remarkable phe- 

 nomenon of the coincidence of the branchial and genital regions. 



Whereas in the craniate Vertebrates the gonads have absolutely nothing to do 

 with the branchial region, in these primitive groups of the Enteropneusta and the 

 Cephalochorda, gonads and gill-slits are, roughly speaking, coextensive. 



The intrinsic importance of gill-slits is abundantly evident from one of their most 

 fundamental properties, namely, their persistency. Whereas teeth, limbs, limb-girdles, 

 digits, etc., after having been once acquired, have been secondarily lost, over and over 

 again, without leaving so much as a trace in the individual ontogeny, gill-slits persist 

 throughout the whole series of craniate Vertebrates, into the human foetus. 



The Memoria tec/mica given below serves to illustrate the position of the Entero- 

 pneusta in the natural system ; and also the dual propensities of this group towards the 

 Echinoderms on one side and the Chordata on the other. The only liberties I have 

 taken are firstly to introduce two new collective names, one of which, Branchiotrema 

 is to include all animals which possess gill-slits at any time in their life-history ; and 

 secondly to apply the name Bilateralia of Metschnikoff somewhat differently from what 

 was originally intended. It is a poor word in any case to apply to a limited group 

 of animals 3 , but its retention recalls the fact that Metschnikoff first discovered the 



1 The substance of the remarks which follow under this heading was given in a paper read before the 

 Cambridge Philosophical Society on Nov. 14th, 1898 (see Proc. Camb. Phil. Soc. Vol. ix., 1899, p. 37.) 



- A. T. Masterman, " On the further anatomy and the budding processes of Cephalodiscus dodecalophus 

 MTntosh." Trans. Roy. Soc. Edinb., Vol. 39, 1898, p. 507. 



Metschnikoff called the Enteropneusta, Bilateralia, and included them with the group of the Echino- 

 derma under the phylum or sub-phylum Ambulacralia. 



