1898.] Dr Willey, The Morphology of the Enteropneusta. 37 



All this indicates onward development; the ideal, in the case 

 of the Loggerhead, is the 16-scuted shell. Those which start with 

 22 perhaps never reach the ideal, but natural selection does not 

 interfere with them. Others start with 21, 20, 19, or 18, and most 

 of these seem to reach the goal. Lastly, there are some few 

 precocious individuals which are born with the right number of 16. 

 Anyhow this is onward development, and since these variations 

 all lie in the direct line of descent (and the more serious the 

 variation, the further back it points) / call this kind of atavistic 

 variation orthogenetic. 



The full paper on this subject, with numerous illustrations, 

 will appear in Pt. III. of Dr Willey's Zoological Results. 



(2) Some points in the Morphology of the Enteropneusta. By 

 Dr A. Willey. 



The body- wall of Enteropneusta is characterised externally by 

 annulations determined by the zonary disposition of epidermal 

 glands and separated by interannular grooves. The potentialities 

 of these structures are indicated by the external liver-saccules of 

 Ptychoderidae which are enlargements of the annulations ; and by 

 the dermal pits of Spengelia which are intergonadial depressions 

 of the interannular grooves. 



In the Enteropneusta and in the Cephalochorda the gonads 

 are more or less coextensive with the gill-clefts, both being 

 primarily unlimited in number. The process of limitation may 

 be supposed to have acted correlatively with the processes of 

 cephalisation and regional differentiation, and the result of it is 

 seen in the concentration both of gonads and of gill-clefts in the 

 anterior region of the trunk which has hence been designated the 

 branchiogenital region. 



A theory of gill-slits was developed, according to which gill- 

 slits arose in the interannular depressions while the gonads were 

 disposed in zones corresponding with the epidermal annulations. 

 The primary function of the gill-slits was the oxygenation of the 

 gonads, their secondary function being the respiration of the 

 individual. In most cases the gonads have been secondarily 

 emancipated from the gill-clefts in correlation with the elaboration 

 of the vascular system. In the author's opinion the evidence in 

 support of this theory is overwhelming. A collective name, 

 Branchiotrema, was introduced to include all animals which 

 possess gill-slits, whether in the adult or in the embryo. 



The theory outlined above will be incorporated, with other 

 matter, in a memoir on Enteropneusta from the South Pacific to 

 appear in Part III. Zoological Results (Cambridge University Press) 

 which will shortly be published. 



