LlNGULID^li.- 
-MOLLUSCA. Tunicata. 
381 
Family— LINGULIDiB. 
The Lingulas (represented on Plate 11, fig. 27, by 
Lingula anatina) have elongated, fleshy oral arms, 
fringed externally with numerous cirrhi, and situated 
on each side of the mouth. The valves of the shell 
of Lingula are thin, depressed, nearly equal, without a 
hinge, but held together by muscles, and supported by 
a thick peduncle, which comes out between the beaks. 
The shell is almost flexible, and is covered with a hard 
olive periostraca. There are seven species known, all 
natives of the seas of warm climates, where they are 
found perforating the mud of shallow bays, or dwelling 
in mud or sandy mud in some of the harbours. 
Class VI. — TUNICATA (The Tunicaries). 
“ The lowest order of Acephalous Mollusca,” says 
Mr. Woodward, “ are called Tanicaries, being protected 
by an elastic tunic in place of a shell. They are ex- 
tremely unlike shell-fish in appearance, and are denied 
a place in most works of conchology ; having no hard 
skeleton, they neither furnish objects for the cabinet of 
the collector, nor materials for the speculation of the 
geologist.” A history of molluscous animals, however, 
would be imperfect without an account of these “ shell- 
less mollusks,” as Cuvier calls them. The Tunicata 
or Tunicaries then, may be characterized as molluscous 
animals which have no true shell, but are enveloped 
in a soft, organized, coriaceous, or gelatinous tunic or 
mantle. This tunic is constructed in the form of a 
sac with two openings, or shaped like a tube, of greater 
or less dimensions, open at both ends ; and is adherent 
by its base, or by a long flexible peduncle, to some 
foreign body, as stones, sea-weeds, &c., or else floats 
freely on the surface of the water. Inside this sac we 
find a second tunic, which is extremely muscular and 
always smooth, while the outer surface is sometimes 
rough and warty. Within this tunic are lodged the 
viscera, consisting of well-defined organs of respiration, 
circulation, and digestion, and a nervous system. The 
respiratory or branchial organ is in form of a sac of an 
oblong, oval, or rectangular shape, and in the Ascidians 
is a large bag of vascular network furnished with vibratile 
cilia, and being porous, allowing the water to pass 
readily through it into the mantle cavity, and thence | 
out by tbe exhalant orifice — (fig. 231). “ The heart is a 
simple, elongated, vasiform muscle, inclosed in a peri- 
cardium, attached to the branchial sac, continued at 
either end into a vessel — the ramifications of one being 
expended chiefly upon the respiratory organ, those of the 
other upon the viscera and tunics of thebody. According 
to the direction of the circulatory currents the one will 
be an arterjq tbe other a vein, and the circulation itself 
will be pulmonic and systematic.” — {Owen.) Some of 
the Compound ascidians have the tunic so transparent, 
that the circulation can be seen going on within. “ A 
very singular condition of the circulatory system has 
thus been detected. The blood actually moves back- 
wards and forwards to and from the heart in the same 
vessels, as it was supposed to ebb and flow in the 
human veins before Harvey’s great discovery.” — 
(Owen.) The digestive organ consists of a stomach ; 
an oblong, dilated sac; an intestine, disposed in a 
sigmoid flexure ; and a simple follicular liver. The 
nervous sj'stem consists of a ganglion, from which can 
be traced certain filaments diverging to each aperture 
of the sac, and others to supply the respiratory sac. 
The two orifices or openings mentioned above as ex- 
isting in the sac or tunic, are branchial and anal. 
Through the upper or in-current, as it is called (the 
branchial orifice), the water enters, carrying with it the 
nutrient particles floating in the water ; these molecules 
are introduced by the action of numerous microscopic 
cilia through this orifice into the esophagus, and from 
that to the stomach. The alimentary excretions and 
the generative products are expelled through the lower, 
ex-current, or anal aperture, by the contraction of the 
muscular tunic. This is so powerful as to enable the 
animals to squirt the water out in a sudden jet. “ The 
only vital actions,” says Professor Owen, “ obvious to 
ordinary vision, are an occasional ejection of water 
from the orifices of the tunic by a sudden contraction 
succeeded by a slow and gradual expansion. Such 
contractions and expansions, aided by the ciliary cur- 
rents which the microscope has detected, and the peri- 
staltic movements of the alimentary, circulating, and 
secerning tubes, are all the actions which the organic 
machinery has to perform in the living Ascidian.” A 
curious discovery was made some years ago in the 
composition of the tunic of these animals. Dr. Schmidt 
ascertained that it contained a large proportion of cel- 
lulose, a ternary organic substance previously believed 
to be peculiar to vegetables. The sexes are united in 
almost all the Tunicata; and the young produced from 
eggs undergo a regular series of changes or meta- 
