RECENT WORK ON THE PROTOCHORDA. 229 



which likewise arise as canalicular extensions of the first gill- 

 pouch into the collar coeloni on each side.^ These truncal 

 canals of Spengelia are such definite structures that I was for 

 a long time perplexed as to their significance. 



I have referred above to the fact that the collar-canals are 

 actively functional ; their walls consist of richly ciliated co- 

 lumnar epithelium, and they retain a uniform calibre from 

 their external orifice to the wide semilunar funnel by which 

 they open into the collar coelom. The truncal canals, on the 

 contrary, taper towards their internal ends, their walls contain 

 ill-defined mucous cells, and, in short, they distinctly appear 

 to be in a vestigial condition. The perihsemal prolongations 

 of the truncal coelom usually contain merely virtual cavities; 

 in other words, their cavities are quite blocked up with mus- 

 cular and connective tissue. This is the case in Sp. porosa, 

 whereas in Sp. alba a true space appears in the posterior 

 portion of the perihsemal cavities, namely, in the region in 

 which the truncal canals occur. 



I can neither state positively that there is an internal open- 

 ing nor that there is not; one thing only is certain, namely, 

 that the truncal canals are there. In Sp. porosa they are 

 longer than in S p. alba, but they present more the appearance 

 of vestigial structures in a chronic state of mucoid degeneration 

 in the former species than in the latter. 



A minute terminal pore is always difficult to find in trans- 

 verse section, or even in any kind of section, and it will be 

 remembered that there was the same difficulty in the case of 

 the atrio-coelomic or brown funnels described by Professor 

 Lankester in Amphioxus. 



If the truncal canals of Spengelia ^ and the brown funnels 



1 This is Spengel's view. Morgan says the collar-pore and first gill-slit 

 arise coincidently. I do not think this affects the present question. Bate- 

 son describes the coliar-pores in B. kowalevskii as arising as thickenings of 

 the outer "atrial" wall which become perforated. The so-called atrial 

 cavities, formed by the overhanging lateral margins of the collar, are peculiar 

 to B. kowalevskii (Spengel). 



' It need be no cause for surprise that these structures only occur in one 

 genus of Enteropncusta. Each species of Enteropneusta may and 



