LAN CELET-FI SUES . 
1219 
give here and their explanations. In the last stages 
the external form of the body is almost complete. But 
it still remains for the month to assume its ventral 
position and its wreath of tentacles instead of the long 
vibratile hairs (not included in our figures) which are 
set during the larval period on a verruciform thicken- 
ing just in front of the mouth-aperture. 
Br and \io stoma besides teaches us the appearance 
presented by the locomotive organs of the vertebrates 
in their simplest form. The locomotive organ proper 
is the hind part of the body, edged above and below 
by a kind of caudal fin or rather a compound of rudi- 
ments of the posterior part of a, dorsal fin, a caudal 
bn, and an anal bn. Before the anus this bn-growth 
Fig. 379. A larva 5 mm. long, taken together with the original of fig. 376. From a preparation briefiy treated with a weak solution of 
osmium bichromate by Prof. G. Retzios and preserved in glycerine. A, from the right; B, from the left. Here the above-mentioned fold (ep) 
originating in the inferior wall of the intestinal canal to form the epibranchial (hyperbranchial) groove has divided into two parts, the one 
(ep') following the tops of the branchial arches, the other (ep") taking its course up towards the roof of the abdominal cavity. In B the 
larval mouth shows the commencement of its alteration and translation from the left side of the body to the ventral margin. A derma] fold 
(or) has laid itself over the upper part thereof; and the superior margin shows signs of dissolution, atrp, atriopore; metcl , edge of right, 
mets, of left metapleure; n), first pair of nerves, with a few ganglion cells (cf. fig. 370, e) visible; other letters as in the preceding figure. 
Fig. 380. Diagrammatical transverse section, A, through a larva of Branchiostoma lanceolatum , with 11 or 12 primary gill-slits, and B, 
through a full-grown specimen of the same species. Enlarged. After Ray Lankester and Willey. 
dm, myoccel of the dorsal fin (that part of the body-cavity (coelom) which belongs to the dorsal fin-lobe); fr, series of fin-rays; m , great 
lateral muscles; my, muscle coelom, myocoel (that part of the body-cavity which belongs to the lateral muscles); nc, spinal cord, with its 
central canal; nch, notochord; d. a., dorsal aorta; v. a., ventral vessel; hit, intestine; sp, splanchnoeoel (intestinal part of the abdominal ca- 
vity), which (in B) is divided by the ingrowth of the atrial chamber into an inner (sp') and an outer (sp") cavity; sp'" (in B), perigonadial 
dilatation of the splanchnoeoel; d. 1. m ., double-layered membrane separating the myocoel from the splanchnoeoel; at, atrium; met , metapleure, one on 
each side of the body, its cavity, according to Ray Lankester, not in open communication with the coelom; s. a. r., floor of the atrial chamber. 
