AMPHIOXTJS LANOEOLA.TUS. 
373 
fifteen myotomes between the atriopore and the anus, and the 
double fin-rays become exceedingly small and terminate before 
the anus is reached. In three specimens counted they were 
developed in relation to the first twelve of the fifteen myo- 
tomes between atriopore and anus ; in one there were thirty- 
four paired rays, in the second thirty-nine, and in a fourth 
forty-one. 
It is not improbable that the double series of ventral fin-rays 
represent a posterior continuation of the same primitive lateral 
fold on each side of the body, which in the anterior two thirds 
of its extent becomes sufficiently large to wrap round the ven- 
tral surface of the body, and by fusion with its fellow along the 
mid-line to form the atrial chamber, whilst posteriorly its line 
of offgrowth has descended on either side, so as to lead to an 
approximation and ultimate fusion in the mid-ventral line, 
forming the double ventral fin. It is of significance in this 
connection that the paired structure of the ventral fin does not 
extend beyond the anus, and that the azygos fin is continued 
ventrally to the right of the anus, whilst anteriorly there is 
a continuity of one lateral fold (the right half of the prseoral 
epipleur or hood) with the dorsal azygos fin. This continuity 
would be similar to that of the azygos fin (passing to the right 
of the anus) with the series of paired ventral fins, if the view 
should be established that the paired fins are the conjoined 
post-atrioporal extensions of the epipleural folds. 
It is important in this matter to distinguish the metapleural 
canals and cartilages from the epipleura upon which they 
develop (see PI. XXXIV, figs. 4 and 5). The metapleura, as 
shown in the figures just cited, are continued posteriorly beyond 
the atriopore and beyond the first two pairs of ventral fin-rays. 
This is evidence in favour of the view that the paired ventral 
fin-rays are continuations of the paired epipleura, for in front 
of the atriopore the area between the two metapleura is formed 
by the fusion of the two epipleura, and it is a legitimate 
inference that behiud the atriopore what lies between the two 
metapleura is also formed by fused epipleura. 
The Number of the Gonad Pouches. — The coelomic sacs in 
