STUDIES ON THE PROTOCHORDATA. 321 



We shall see, however, that the difference in the dimensions 

 of the primary stigmata was misinterpreted by van Beneden 

 and Julin, and that the irregularity in their order of formation 

 does not really exist. 



Krohn (23), in a paper remarkable for the number and 

 comparative accuracy of the observations contained in it, on 

 the 'Development of Phallusia mammillata,' Cuv., de- 

 scribes and figures a young individual with two gill-slits on 

 each side, and describes accurately for the first time the 

 two atrial apertures, which afterwards fuse into one. The 

 first two gill-slits appear simultaneously. He says, further 

 (p. 326), ^' Auf diese zwei Paar Oeffnungen die sich mittler- 

 weile verlangern und spaltenahnlicher werden, bleibt der 

 Athemsack noch langere zeit hindurch beschrankt. Endlich 

 entstehen jederseits in der Briicke zwischen ihnen zwei 

 neue Oeffnungen und bald darauf tritt hinter der urspriing- 

 lichen hinteren Spalte noch eine Oeffnung hinzu so dass die 

 zahl sammtlicher Oeffnungen jetzt schon funfe betragt." 



In this description we shall see that Krohn came nearer the 

 truth than any other investigator up to the present. His 

 account of the mode of origin of the transverse rows of stig- 

 mata is wrong, as has been already shown, at least with 

 reference to the first six rows, by van Beneden and Julin. 



Commencing, then, with a stage at which two branchial 

 apertures are present on each side (PI. XXX, figs. 2 — 4), we 

 find that they elongate considerably in the transverse direc- 

 tioUj and meanwhile, as time goes on, become curved in 

 opposite directions^ so that their concave sides face each other 

 (fig. 6). 



The next and most important stage shows us that from the 

 inner extremities — that is, from the ends nearest the endo- 

 style of each of the two original slits — a small portion is in 

 process of being constricted off (figs. 5 and 7 — 10). The 

 various stages in this remarkable and hitherto undescribed 

 mode of formation of the two intermediate slits are shown in 

 PI. XXX, figs. 7 — 10. These intermediate apertures, then, 

 are what Krohn saw after, but not during their formation : 



