BALANOGLOSSUS. 437 
Respiratory and vascular systems.—The respirator} 
system consists of many pairs of ciliated gill-slits. They 
open dorsally by minute pores behind the collar. In 
development they begin as a pair, increase in number from 
in front backwards, and they go on increasing long after 
gs an 
Uv.Y. un, 
Fic. 23 speereide digs section through gill-slit region of 
Ptychodera minuta.—After Spengel. 
The section, somewhat diagrammatic, shows a gill-slit (g.s.) to left, 
and a septum between two slits to the right; @.z., dorsal 
nerve; d.z., dorsal vessel; v.7., ventral nerve; v.v., * ventral 
vessel; £., hutritive part of gut; g-1, respiratory part of gut; 
¢., lateral coelomic spaces; ¢.77., longitudinal muscles; 2., 
reproductive organs. As the gill-slits are oblique, the whole of 
one could not be seen in a single cross-section. 
the adult structure has been attained. Water passes in by 
the mouth and out-by the gill-slits, where it washes branches 
of the dorsal blood vessel. 
There is a main dorsal blood vessel, which, at its anterior 
end, forms a heart lying adove the notochord, and below 
a closed contractile dilatation, sometimes called the “ peri- 
cardium.” Beside the latter there is a paired ‘“ proboscis 
gland,” formed from the ccelomic epithelium. There is a 
ventral vessel beneath the gut; and numerous smaller 
vessels. The almost colourless blood flows forwards 
dorsally, backwards ventrally. This system should be 
contrasted with that of Amphioxus. 
Excretory and reproductive systems.—No nephridia are 
