1218 
SCANDINAVIAN FISHES. 
wards and forwards over the tops of the branchial slits. 
The latter {end, with its supporting rods, the so-called 
endostyle , with a right and a left part), on the other 
hand, follows, according to Willey, an opposite direction 
of development, backwards from in front, originating in 
a thickening, at first transversal (figs. 375 — 379, end), 
of the intestinal wall beside the club-shaped gland. 
The first rudiment of the atrium appears (fig 380, A, 
sar ) where the atriopore afterwards has its place, and 
presents the appearance of two horizontal ridges, grow- 
m 
VII 
Fig. 376. A larva 3 1 /., mm. long, taken swimming freely in the sea, between 1 and 4 fathoms below the surface, at Kristineberg (Bohus- 
lan), August 17, 1894. Seen from the left. Letters and numerals as in the preceding figure; besides: am, the ciliated mouth of the cerebro- 
spinal canal; ep, the forward growth on the inside of the intestine, a fold originating from the inferior wall of the intestinal canal. Number 
of the myomeres here 61. Gill-slits and their rudiments visible here to a number of 13 (7 — XIII). 
basket. Some of the first-formed gill-openings close 
again; and the posterior part of the branchial basket 
has its slits symmetrically arranged after the termina- 
tion of the larval period. The mouth appears on the 
left side, first as a fine slit, afterwards as a large, el- 
liptical aperture with tumid margin (fig. 375). A true 
anus opens, also, as a rule, on the left side, and the 
original gastrula mouth grows together, the connexion 
between the intestinal canal and the cerebrospinal canal 
being thus cut oft’. As yet the larva moves only with 
ch 
imt ! 
the aid of the dermal cilia; but it has begun to bend 
its body, since the rostral and caudal fins have become 
sharpened. Over the gill -slits there grows from above 
(fig. 377) a longitudinal fold from the walls of the 
body (somatopleures, the outer parts of which grow 
into the so-called metapleures or abdominal fins, met, 
Fig. 377. Forepart of a larva 3 1 / 2 mm. loug, seen from the right. 
After Ray Lankester and Willey. Here it appears how the wall of 
the body from the ninth gill-slit forward lies like a leaf outside the 
upper parts of the slits, and further forward is continued in a sharp, 
free edge, forming an upward curve on the right side of the head. 
Letters and numerals as in the preceding figure. 
figs. 379 and 380) on each side, first on the right; 
and between these folds the atrium is formed, first only 
on the ventral side and growing forwards from behind. 
The construction of the branchial basket is continued 
(figs. 376, 378, and 379) by the addition of epibran- 
chial and hypobranchial grooves. The former {ep) ori- 
ginates, so far as we have been able to see, as a fold- 
like incurvature, advancing in its development forward 
from behind along the lower wall of the intestinal canal, 
which at the hindmost rudimentary gill-slit grows up- 
ing to meet each other (fig. 380, B), on the inside of 
the above-mentioned dermal folds {met), whose inferior 
margin subsequently becomes on each side of the body 
a longitudinal lateral ridge (abdominal fin or meta- 
pleure). Afterwards the atrium expands upwards, en- 
croaching upon the region of the abdominal cavity on 
fr VII- !'■ ’■ cil fr 
Fig. 378. Anterior end of a somewhat larger larva, seen from the 
right. After Willey. Here a series of seven secondary gill-slits 
(/- — VII') has appeared on the right side, above the inferior ends 
of the primary slits (7 — XIII), which have been translated for the 
most part to the left side, and among which the thirteenth (hindmost) 
has been obliterated, and the first has almost suffered the same fate. 
The endostyle (end) has grown backwards, and its posterior extremity 
extends behind the club-shaped gland (gl). Further forward the velum 
of the mouth cavity ( vel ) has begun to develop. In the skeletogenous 
layer (chordal sheath) a series of so-called fin-rays (fr) has appeared. 
On the atrial floor two so-called renal organs (r) may be seen. Letters 
and numerals otherwise as in the preceding figure. 
the sides of the branchial basket, and continued within 
the abdominal cavity even behind the atriopore, beside 
the intestine. 
The details of this development should be easy of 
comprehension with the aid of the figures which we 
