Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/1237/
TUNICATA. 1227 
pying the common mass, of which it becomes 
a new inhabitant. The communication be¬ 
tween the mother and the young animal be¬ 
comes obliterated ; but for some time yet, all 
the young individuals growing from the same 
branch remain united by their pedicle, and it 
is this union, apparently, that determines their 
mode of grouping into “ systems.” * In JOi- 
demnum gelatinosum, the buds growing on 
these proliferous stolons are very different 
in appearance from the ova expelled by 
the animals ; for not only did they differ in 
aspect and form, but their bulk is at first 
twenty or thirty times less than that of the 
vitelline mass of the ova. In the Amaroucium 
proliferum, Milne-Edwards has frequently 
found on the surface of a rounded mass, formed 
by a colony of these animals, many little 
filiform twigs, simple or branched, formed by 
a prolongation of the common tegumentary 
substance, and consisting of a tube closed at 
the end, and enclosing, in its interior, one or 
more embryos in different states of develop¬ 
ment. These young individuals terminated 
inferiorly by a peduncle, prolonged in form of 
a slender tube into the common mass, and 
springing apparently from the abdominal tunic 
of an adult individual. This mode of propa¬ 
gation by buds, which the compound Ascidians 
possess in common with the Polypifera, is, as 
we have above described, found in the Clavel- 
linidæ ; the only important difference being, 
that in the latter the tegumentary envelope of 
the young is not so largely developed as in 
the Botryllidæ, and does not become fused 
with that of the adults; whence it results that 
the individuals springing from the same stem 
remain isolated, instead of being united into 
a common mass. 
Anatomy of Pyrosoma. — The common 
tegumentary mass of Pyrosoma is semitrans¬ 
parent, subcartilaginous, toughish, and some¬ 
what extensile. The exterior of the hollow, 
conical, or cylindrical body, formed by the ag¬ 
gregation of the individual Pyrosomata, is co¬ 
vered with numerous elongated tubercles, of a 
rather firmer consistence than the rest of the 
mass. Each of these constitutes one extremity 
of an individual member of the living group. 
The opposite extremity opens into the cavity of 
the cylinder, and is not free, but, like the trunk 
of the individual’s body, is closely connected 
by the common mass with the similar parts of 
other individuals lying above, below, and on 
* M. Savigny figures a nascent system, originated 
apparently by this grouping of the buds ; and 
Professor Van Beneden coincides with M. Milne- 
Edwards in the above view of the subject, but M. 
Steenstrup expresses his opinion that the mode in 
which the colonies and systems of the Botryllidæ are 
formed, is not sufficiently explained by this hypo¬ 
thesis; and, although (following Milne-Edwards) 
he considers M. Sars to have been misled in re¬ 
garding the ova of the Botrylli as producing 
groups of animals, yet he is inclined to consider this 
grouping to be really a fœtal condition, but occur¬ 
ring in some hitherto unnoticed “ aggregate ” form 
of animal, produced from the “ solitary ” larvæ de¬ 
scribed by Milne-Echvards,justasthe solitary Salpæ 
bring forth Salpa-chains. 
either side of it. In some species the animals 
are arranged much more regularly than in 
others, and appear to form piled-up rings or 
circles of individuals, more or less analogous 
to the otherwise disposed circular systems of 
some of the Botryllidæ. In Pyrosoma atlan- 
ticum the tubercles are simply conical, and 
are perforated terminally. In P. elegans, also, 
the external orifice of the individual opens at 
the extremity of the tubercle, and through it 
the water contained in the great cylinder has 
been seen to escape freely in little jets, when 
the Pyrosome has been taken out of the sea. 
In P. giganteum the tubercles are of various 
sizes, some being short and indistinct, and 
others, on the contrary, very much deve¬ 
loped. The largest are conico-cylindrical flat, 
and lanceolate at the extremity, with the 
minute branchial orifice on the inferior aspect. 
This lanceolate extremity is crenulated on its 
sharp edges, and presents on its inferior aspect, 
between its point and the aperture, a slightly 
prominent keel. The branchial orifice is 
sometimes surrounded by a slight, free, crenu¬ 
lated membrane. 
The interior of the great cavity is generally 
smooth. Its walls are perforated by the 
numerous minute anal orifices of the com¬ 
ponent individuals ; and, at a slight depth, its 
surface is studded with a great number of 
yellowish, rose-coloured, or carmine spots, 
which are the hepatic and other visceral 
organs of the numerous animals. The ter¬ 
minal aperture of the large, conical, compound 
body of the Pyrosoma has, according to Le- 
sueur and Savigny, a membranous border, 
which can be sometimes drawn together so as 
to close the cavity ; and Mr. F. D. Bennett 
observed that, when first removed from the 
sea, the broader extremity of the cylinder pre¬ 
sented a wide and circular orifice, forming 
nearly a continuous surface with the central 
tube ; but when the animal was kept in a 
vessel of sea-water, or much handled, this 
orifice was closed by the contraction of a 
smooth, dense membrane at its margin, and 
which either obliterated the aperture, or left 
but a minute central orifice; water at the 
same time being contained in the barrel or 
tube of the body. 
Besides the common envelope or test, each 
individual animal has an inner tunic or man¬ 
tle. This is a very thin, delicate membrane, 
attached apparently at four points only, two 
of which are at the extremities; that is, at the 
branchial and anal orifices ; and the other two 
are at two rounded, compressed bodies, one 
on either side, just beyond the anterior mar¬ 
gin of the branchiae, and regarded by Savigny 
as ovaries. 
The branchiæ line the inner surface of this 
inner tunic. They are oval in form, and their 
dorsal borders meet each other, and are at¬ 
tached along the dorsal aspect of the mantle ; 
but they are separated, at their anterior and 
ventral borders, by a considerable space, which 
is partly occupied by the ventral sinus (fig. 
786. i,i). The branchial tissue is traversed 
by numerous vessels anastomosing with each
        

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