298 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



A. Evidence of unlimited Gill-slits. 



The evidence is of various kinds and derived from various sources, e.g. number, 

 formation, limitation and vestiges of gill-clefts. 



a. Number and formation of gill-slits. It is a fact which sharply distinguishes 

 the Enteropneusta and Cephalochorda from the craniate Vertebrates that new gill-slits 

 are added at the posterior end of the pharynx throughout life. 



In spite of this successive addition, at longer or shorter intervals, there is in 

 most cases a maximum which is usually not overstepped before death ensues. In 

 Amphioxus the maximum may be taken as from 90 to 100 gill-slits on each side. In 

 Pt. flava the number of gill-clefts acquired by the macrobranchiate forms may be 

 taken at about 150 pairs. In Pt. aurantiaca, according to Spengel, the gill-slits may 

 reach the impressive total of 700 pairs. In this case it appears to be impossible to 

 assign a maximum. Pt. minuta goes to the other extreme and never has more than 

 40 pairs (Spengel). 



/3. Limitation of gill-clefts. The fact of limitation is shown in absolutely un- 

 equivocal manner within the group of the Enteropneusta. It is implied in the facts 

 given in the preceding paragraph which might easily be supplemented. It is however 

 sufficient to compare the conditions met with in Pt. flava and Pt. ruficollis respec- 

 tively. Whereas the length of the pharynx of Pt. flava varies enormously, namely, 

 from less than a centimetre up to about 3 centimetres, that of Pt. ruficollis is re- 

 markable both for its shortness and its constancy (cf. PI. XXVI. Figs. 1 and 2; and 

 PI. XXVII. Fig. 7). 



If further demonstration of limitation be required it is furnished in a totally 

 unexpected way by the postbranchial canal of Pt. ruficollis as compared with the 

 corresponding portion of the gut in Pt. flava and Pt. camosa. In the two last- 

 named species the last pair of gill-slits occurs at the dorso-lateral margins of this 

 structure and new slits are duly added in line with the pre-existing slits. In Pt. 

 ruficollis the gill-slits have nothing whatever to do with the vestigial postbranchial 

 canal, the last few pairs opening at the base of it directly into the ventral division 

 of the gut. This faculty of the gill-slits of shifting their position and having their 

 primary topographical relations radically changed is worthy of particular note. At the 

 posterior end of the pharynx in Pt. ruficollis the gill-slits have, in effect, moved from 

 a dorsal to a more ventral position (PI. XXX. Figs. 32, 33). 



7. Vestiges of gill-clefts (see also below p. 321). Under this head are probably 

 to be placed the intestinal pores (Darmpforten) originally discovered in Balanoglossus 

 mereschkoivskii by Schimkewitsch, whose observations were greatly extended by Spengel. 

 They occur in certain species only, namely, Schizocardium brasiliense, Glandiceps talaboti, 

 Ol. hacksi, Bal. kowalevskii and Bal. mereschlcowskii 1 , and in my Spengelia alba (see 



1 Spengel says Bal. mereschkou-skii (the White Sea Enteropneust) is probably co-specific with Bal. koica- 

 levskii. 



