AMPHIOXUS LANOEOLATUS. 
375 
myotomes, but that this relation is not subsequently main- 
tained. 
The accounts of the late larval condition of Amphioxus are 
not sufficiently satisfactory to enable us to formulate a very 
definite conclusion as to this early relation of the gill-slits to 
the myotomes. It is, however, quite certain that after the 
larval phase all relation between the number of the myotomes 
and the number of the gill-slits is lost. The gill-slits go on 
increasing in number by addition at the posterior end of the 
series throughout the period of growth — probably as long as 
the animal lives — whilst the full number of myotomes is 
acquired at a very early period, and is not subsequently 
increased. Owing to this fact it is possible in any Am- 
phioxus to observe the mode of formation of the gill-slits, and 
it is found that they originate as oval or nearly circular per- 
forations of the proper body wall, which become divided each 
into two by the growth from the dorsal margin of the oval slit 
of a longitudinal bar or tongue, comparable to the tongue of a 
Jew’s-harp, which thus divides each primary slit or gill 
aperture into two. 
The tongue bars can be distinguished throughout the series 
by the fact that they are supported by a hollow chitinous rod, 
whilst the adjacent bars separating primary slits from one 
another are solid (PI. XXXYIjB, figs. 1 and 2). Also the 
primary bars are provided with a plate-like projection on their 
external border which becomes deeper dorsalwards and shal- 
lower ventralwards. This plate-like projection is soft-walled 
and hollow, containing a space which communicates with the 
“ dorsal ” or supra-pharyngeal coelom. In my earlier paper 
(12) I called these soft plate-like projections the pharyngo- 
pleural folds. In the more dorsal or upper part of the bars 
the pharyn go-pleural folds are so deep as to rest for some 
distance against the inner face of the down-grown epipleura. 
In consequence of the oblique and almost horizontal position 
of the bars and slits throughout the middle third of the per- 
forate region of the body, and, in consequence of the adhesion 
of the pharyngo-pleural folds to the epipleura, the atrial 
