386 Salpid^. MOLLUSOA. Salpid^. 
were very indefinite, presenting much of the appearance the surface perfectly resembled incandescent cylinders 
of great masses of fire, or rather of enormous red-hot of iron.” The genus is represented by fig. 232 
cannon balls ; whilst those more distinctly seen near j {Pyrosoma giganteum) 
Order— MONOCHITONIDA. 
Inner sac adhering throughout to the external tunic. 
Orifices without tentacular fringes. 
This order contains two families — one of which is 
always free and floating; the other free also, but living 
buried in mud. 
Family I. — SALPIDA3. 
The Salpce are distinguished by their outer tunic 
being thin, elastic, elongated, compressed, and open at 
both extremities, the two orifices being opposite each 
other.^ They are free, and habitually swim on the 
waters of the ocean, and are alternately solitary and 
aggregated. The solitary individuals resemble short 
but rather wide tubes, which are often of considerable 
size, and so transparent that, as Professor Forbes says, 
they look as if carved in crystal. 
The history of these Tunicaries is replete with interest, 
especially on account of their singular mode of repro- 
duction. These occur under two conditions; at one 
time appearing, as described above, in solitary indivi- 
duals, floating freelj" on the surface; at others, asso- 
ciated into lengthened groups, or chains, composed of 
numerous individuals of similar form, and swimming 
with regular serpentine movements through the tran- 
quil waters. The individuals which compose these 
chains contract and expand simultaneously, “ keeping 
time, as it were, like a regiment of soldiers on parade,” 
each chain thus appearing to be a single being, acting 
through the influence of a unique will. After floating 
for a certain time under this form the societjf is broken 
up, and each individual, so separated from the chain, 
propagates a young one like itself. This individual 
grows to the size of its grand parent, and then brings 
forth, in its turn, a social chain of young Salpce, which, 
again, give origin to the solitary individuals ; thus, as 
Chamisso discovered, for it is to him we owe the know- 
ledge of this curious fact, chained Salpce do not produce 
chained Salpce, but solitary Salpce, and these do not 
reproduce solitary, but chained Salpce; or, as this 
naturalist e.xpresses it, “ a Salpa mother is not like its 
daughter or its own mother, but resembles its sister, 
its granddaughter, and its grandmother !” the alternate 
generations only resembling each other. As the mem- 
bers of one generation are very dissimilar from those of 
the other, the species described previously to this dis- 
covery of Chamisso’s, were accordingly very much mul- 
tiplied. The only conspicuous vital action in these 
Salpce, says Professor Owen, are the rhythmical contrac- 
tion and expansion of the mantle, in which the elasti- 
city of the outer tunic antagonizes the contraction of 
the inner one. During expansion the sea-water enters 
by the posterior aperture, and is expelled, in contraction, 
by the anterior one ; its exit by the opposite end being 
prevented by a valve. The reaction of the jet, which 
is commonly forced out of a contracted tube, occasions 
a retrograde movement of the animal. The currents 
which successively traverse the interior of the animal, 
renew the oxygenated medium upon the surface of 
the respiratory organ, bring the nutrient molecules 
within reach of the prehensile, subspiral, labial mem- 
brane of the mouth, and expel the excrements and 
generative products. Thus a single act of muscular 
contraction is made subservient, by the admirable co- 
adjustment of the different organs, to the performance 
of the functions of locomotion, nutrition, respiration, 
excretion, and generation.- The Salp/ce are found in 
some of our northern seas, but they are more abundant 
in the Mediterranean, and in the warmer parts of the 
ocean. In these localities they are remarkable for the 
brilliant phosphorescent light they give out at night, 
especially wdien the water is agitated. The long chains 
of these animals swimming near the surfirce, have been 
described as occasionally producing the effect of long 
ribands of fire drawn along by the currents. 
Three genera have been enumerated. 
Genus Salpa. — This genus is the typical and most 
important one, and may be defined in the solitary 
form, as oblong, subcylindrical, truncated in front by 
the oral orifice, pointed posterior- 
ly, anal orifice subterminal, each 
orifice furnished with a valve; test 
thin, transparent; muscular man- 
tle incomplete, forming a set of 
transverse or oblique bands; man- 
tle cavity lined by a system of 
vascular sinuses ; gill rudimen- 
taiy, consisting of two narrow, ob- 
lique, unequal laminae, forming an 
oblique band across the interior; 
visceral mass or nuclens situate 
near the posterior extremity, and 
veiy conspicuous,, owing to the 
brilliant orange, brown, or reddish 
hues of the liver. The sexes are 
combined, and all the species yet 
observed are viviparous ; the 
young being produced by gem- 
mation in chains consisting of 
individuals unlike the parent, 
and becoming oviparous. The individual Salpce vary 
in length from half an inch to tei inches, and the chains 
are found extending from a few inches to many feet. 
A good many species have been described. The genus 
is represented by fig. 233 {Salp z octofora). 
Genus Doliolum. — This genus was e.stablished for 
some little animals nearly allied to the Salpa, which 
are described as round, or in the form of a little cask, 
transparent, open at the ends, and from two to ten 
lines in length. The method of progression in these 
Fig. 233. 
