46o NATURAL SCIENCE. Dec. 



indicate its degenerate character. But now, thanks to the investiga- 

 tions of several workers, we are in full possession of the details of the 

 excretory system, which has been proved to be constituted on the 

 plan of that of other Vertebrata, but, of course, with differences in 

 details. The excretory tubes of the Aniphioxus appear to represent 

 the " head-kidney " or pronephros of other vertebrates. But they 

 have no continuous longitudinal duct such as exists in all other types. 

 The place of this is, however, physiologically taken by the atrial cavity, 

 into which the series of tubes opens. It is now the general opinion 

 that the longitudinal duct of the pronephros in the higher vertebrates 

 is a formation from the epiblast — a groove to begin with, which is 

 later converted into a tube ; since the atrial cavity is also an epi- 

 blastic formation, no great gap is indicated by the absence of a special 

 longitudinal duct. 



It is, however, rather the comparisons with lower forms of life 

 that are so interesting in Amphioxus. The connecting link between 

 vetebrates and invertebrates has been long sought, and in spite of 

 Amphioxus, Ascidian, Balanoglossiis, or even the latest claimant, Cepha- 

 lodiscns, it has not yet been found. For as all the animals which have 

 been mentioned show the three essential Chordate characters, the 

 dorsal nervous system, the notochord, and the gill- slits, we are really 

 no further than we were before, and there is still hope for the 

 Annelid to put in a claim which will be admitted. Mr. Willey gives, 

 in his introductory remarks, an excellent resume of the celebrated 

 opinion of Geoffrey St. Hilaire upon the matter. The boldness — yet, 

 so far as it went, logic — with which that great naturalist compared 

 the exoskeleton of an insect with the ribs and vertebral column of a 

 fish, have been equalled, if not surpassed — at any rate, so far as 

 regards the boldness — by modern comparisons of Crustaceans and 

 Limulus with vertebrates. Of the three essential chordate characters, 

 the gill-slits would seem to be the hardest to account for ; the 

 existence of breathing-organs as mere processes of the skin seems 

 so natural as it is very widely spread. There has, however, been 

 a suggestion made, which is duly quoted by Mr. Willey, and which 

 seems to shed at least a pin-point of light upon the matter. Mr. 

 Harmer and Mr. Brooks have thrown out the hint that the 

 function of the gill-slits was originally that of getting rid of the super- 

 fluous water ; a form like CepJialodiscns, with copious arborescent 

 gills, would appear to have no need of aerating the blood in the 

 other, so to speak, far-fetched manner. And yet Cephalodiscus has 

 gill-slits. The only invertebrates not within the charmed circle of 

 the forms already referred to, which are seriously considered by Mr. 

 Willey, are the Echinoderma and the Nemertines. The former 

 group come in, of course, through the likeness of their larva to the 

 Toynai'ia larva of Balanoglossus, as was fully explained in our October 

 number, on p. 245 of this volume. The Nemertines have a proboscis 

 sheath, which Professor Hubrecht compared -to the notochord, and a 

 pair of nerves which tend to become approximated dorsally, and 

 therefore to occupy the position of the central nervous system of the 

 vertebrate. Mr. Willey, however, at the conclusion of his book, 

 feels unable to do more for the vertebrate pedigree than to suggest 

 " that the proximate ancestor of the vertebrates was a free swimming 

 animal, intermediate in organisation between an Ascidian tadpole 

 and Amphioxus. . . . The ultimate or primordial ancestor of the 

 Vertebrates would, on the contrary, be a worm-like animal, whose 

 organisation was approximately on a level with that of the bilateral 

 ancestors of the Echinoderms." 



