L A N C ELET-FISHES . 
1215 
in the hypobranchial groove. Langerhans described 0 
as sensory cells a number of cylindrical epithelial cells 
scattered in the skin and most numerous on the fore- 
most part of the body, furnished distally with a long, 
stiff hair, proximally with a process to which the peri- 
pheral end of a nerve fibrilla attaches itself. In the 
vertical dermal fin around the anterior end of the body 
Vogt and Yung remarked 6 small, round or oval, trans- 
parent ganglion cells, situated oftenest in the fork be- 
tween .two nerve twigs (fig. 370, e ). 
These organs of sense have indeed scarcely been 
submitted as yet to adequate investigation; but so much 
is known, that the rudimentary eye has the same struc- 
ture as in the lower Polychtetous worms, and the ol- 
factory depression also finds its nearest anatomical homo- 
logue in Nemertines, Turbellarians, and some other 
worms. There is this difference, however, that in 
Branchiostoma both these organs are single, and that 
the olfactory organ is asymmetrical, situated on the left 
side of the body. The only further reason which might 
be adduced foi* calling the anterior end of Branchiostoma 
a rudimentary head, is that the spinal cord, as men- 
tioned above, dilates in front, though but slightly, into 
a vesicle from which there originate two fairly sym- 
metrical pairs of nerves, the second pair larger and 
more numerously ramified than the other spinal nerves. 
It is therefore not surprising that some have proposed 
to separate tins animal into a distinct class, and have 
said: it is neither a vertebrate nor an invertebrate, it 
is something between the two. 
The organs of generation (fig. 364, c) are a row 
of saccate vesicles, free from each other and usually 
numbering 26 — 28 on each side in full-grown indivi- 
duals, between the abdominal wall and the atrium. The 
male and female can be distinguished only by the con- 
tents of the vesicles 0 ; and the sexual products are pro- 
bably emitted, as a rule, through the atrial pore. Ivo- 
walewsky 07 and Hatschek 0 , however, saw these pro- 
ducts discharged through the mouth aperture; and one 
of the females received by us from Bohuslan had several 
ripe eggs in the mouth cavity, where they may easily 
arrive, as Hatschek has pointed out, after first being 
liberated, by the bursting of the generative sacs, in the 
atrium and then passing, through the clefts of the 
branchial sac, into this cavity and out at its anterior 
(sphincterial) aperture. 
As excretory organs in the full-grown Branchio- 
stoma Weiss has adduced' a large amount of the epi- 
thelial lining of the atrium, especially on two tubiform 
prolongations of the said cavity which project into the 
abdominal cavity proper or coelom (atrio-coelomic fun- 
nels). In this connection he also draws attention to his 
own discovery of small, tubular glands (kidneys?) si- 
tuated at the top of the branchial tongue-bars (secon- 
dary cartilaginous rods) and opening on the outer side 
thereof into the atrium. 
The character of the genus and its relation to other 
animals are best explained, however, by the history of 
evolution 87 . Branchiostoma deposits its ova (fig. 371, A), 
Fig. 371. A , an unimpregnated egg, with the so-called polar body 
(Richtungskorper, R), a small body liberated from the egg and marking 
that pole thereof at which the segmentation is most active. B, an 
impregnated egg, with two cleavage-cells not yet quite divided from 
each other. C , an egg segmented into 8 parts. D , an egg with 8 large 
cells round the inferior pole and 5 sixteen-celled circles higher up. E , 
optical section at the Blastula stage (simple sacculation), with the in- 
ferior pole flattened, the commencement of its upward invagination 
towards the inside of the wall of the upper pole. F, optical section 
of an embryo in the Gastrula stage, with the above-mentioned in- 
vagination completed. Each cell of the ectoderm (EM) is furnished 
with a flagellum. Ent, entoderm, answering to the lower part of the 
Blastula wall in the preceding stage. The Gastrula mouth ( Gm ) is 
the downward opening. After Hatschek. X 140. 
0 Arch. Mikrosk. Anat., Bd. XII (1875—76), p. 303. 
b Lehrb. prakt. verrjl. Anat., Bd. 2, p. 361. 
c And also, according to Vogt and Yung (1. c., pp. 375, 376) by the inner epithelial investment of the transverse ventral muscle, 
this being developed into a kind of support for the organs of generation (deferent grooves?), different in males and females. 
d Mem. Acad. Sc. Petersb., ser. VII, tome XI, No. 4, p. 1. 
e Sturl. iib. Entw. d. Amphioxus , Arb. Zool. Inst. Univ. Wien, tom. IV, H. 1, p. 14. 
f Quart. Journ. Micr. Sc., n. ser., No. CXXIV (Nov. 1890), p. 489. 
^ Kowalewsky, 1. c., and Arch. Mikr. Anat., Bd. XIII (1876 — 77), p. 181; Hatschek, 1. c., Ray Lankester and Willey, Quart. 
Journ. Micr. Sc., n. ser., No. CXXIII (Aug. 1890), p. 445. 
Scandinavian Fishes. 
153 
