THE STRUCTURE AND AFFINITIES OF THE SEA-SQUIRTS. 245 
that it was in them that Chamisso * discovered a peculiar mode 
of reproduction, termed by him ‘‘alternation of generations,” a 
discovery since verified by the late observations carried on by 
Huxley on board the “ Eattlesnake.” 
A Salpa (fig. 11) is a somewhat irregular, hollow, translucent 
cylinder, open at both ends, each orifice being furnished with a 
valve. Though the animal swims “ indifferently,” says Huxley, 
“ with either end forward, and with either side uppermost,” that 
end where the mouth opens may be considered as anterior, and 
that side as dorsal where the heart is situated. 
There are two tunics, adherent only at the margin of the two 
orifices. The outer corresponds to the Ascidian test, while the 
inner is made up of the homologues of the mantle and lining 
membrane, which are in close contact, except where blood- 
channels intervene. (Fig. 11, M and T.) 
A few bands (five or seven in number) of striped muscular 
tissue run across the inner tunic, transverse to the long axis 
of the body {hm. hm, fig. 11). A band (“ hypopharyngeal 
band,” Huxley), composed of two laminae adherent along their 
dorsal edge, crosses the body-cavity (“pallial chamber”) obliquely 
from behind forwards and downwards (br, fig. 11). This has 
been called a gill, but “ somewhat too exclusively, as there can 
be little doubt that the whole respiratory cavity performs the 
branchial function.” 
A tongue-shaped body, the “ languet ” (“langlicher Organ,” 
Eschricht), at the base of which lies a ciliated sac, projects 
into the body-cavity, where its ventral wall is joined by the an- 
terior end of the so-called “ gill.” It is supposed to subserve 
the sense of taste. (Fig. 11, cci) 
A single ganglion lies just behind the languet, to which, as 
well as to the walls of the body, it sends nerves. To its lower 
surface is attached a vesicle containing pigment and calcareous 
bodies, leading to which is a depression in the outer tunic. 
(Fig. 11, g.) This probably corresponds to the auditory organ 
in those orders of which the snail and river-mussel are repre- 
sentatives. 
The digestive tract is very simple, being included in a small 
knob — the “ nucleus ” — situated at the hinder end of the body. 
(Fig. 11, 
It is connected with the mouth by a furrow running along 
the ventral aspect of the animal. The intestine is spirally 
coiled, and has a sac — the stomach — attached to its left side. 
The anus opens above and to the right side of the mouth. Over 
the last portion of the intestine is spread a network of trans- 
* Better known, no doubt, to most of oiir readers as the author of “ The 
Shadowless Man ” (Peter Schlemihl), 
