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/lit29465/1233/
TUNICATA. 1223 
At each side of the base of the group of ten¬ 
tacles, a little prominent lobular process is 
present. Lastly, the side of the body, opposite 
to that on which the tail is placed, becomes 
somewhat strongly embossed near its pos¬ 
terior extremity, and towards the space where 
the above-mentioned black points occur {fig. 
784. b). 
The ovum ready for exclusion differs ap¬ 
parently little from the foregoing state. The 
two anterior styliform appendages have almost 
entirely disappeared, and the three obtuse 
processes are further developed. The trunk 
is contracted towards its anterior extremity, 
and the yolk-mass still further condensed at 
the centre of the ovum. 
The external membrane of the ovum becomes 
excessively thin, and then breaks and allows 
the embryo to escape. Generally this ex¬ 
clusion takes place in the interior of the cloaca, 
but sometimes not until the ova have passed 
out by the anal orifice. However that may 
be, the young animal, free from its envelopes, 
soon extends its tail, and swims in the ambient 
fluid by the aid of its undulatory movements. 
In its general form the young Ascidian re¬ 
sembles somewhat that of a newly born tad¬ 
pole {fig. 784. c) ; but it still more resembles a 
Cercaria. The trunk or body of the larval Poly- 
clinuvi is oval and rather depressed. The 
whitish tissue of the future integument occupies 
all the surface, and is considerably developed 
at the margins ; its substance is granular and 
apparently subgelatinous ; its consistence is 
greatest at the surface; and it does not appear 
to possess a membranous investment. To¬ 
wards the centre of the trunk is a large ellip¬ 
tical membranous sac, the internal tunic of 
Milne-Edwards ; this is filled with the yellow 
substance of the vitellus, and is continuous 
anteriorly with the three tubes dilated at the 
end, and terminating at the anterior wall of 
the egg in a sort of sucker. By means of 
gentle graduated pressure, some of the yolk 
may be easily made to pass from the principal 
sac into these appendages, and vice versa ; the 
little capsule, also, terminating each of these 
appendages can be made to protrude exter¬ 
nally by the same means. At the base of these 
three processes the vestiges of the others 
formerly occupying the intervening spaces may 
be observed. The yellow substance contained 
in the internal tunic appears to be separated 
into two portions; the one is clear, and situated 
near the wall of this sac, and the other, denser 
and of a deeper tint, occupies the centre. Pos¬ 
teriorly a little marginal space, clearer than the 
neighbouring parts, is also visible, and on one 
of the sides the above-mentioned minute black 
spots are visible. The tail is very large, and, 
like the trunk, is composed of two distinct 
parts ; the one superficial, colourless, diapha¬ 
nous, gelatinous, and much resembling the al¬ 
bumen of the eggs of frogs ; the other, central 
and yellow. This latter part is continuous 
anteriorly with the central sac of the trunk, 
and is also composed of a membranous tunic, 
enclosing a yellow granular and semi-fluid 
substance. It sometimes appeared to have a 
central canal. The larvae, after swimming about 
with an active wriggling motion for a few hours, 
attach themselves to the surface of a solid 
body, and, if disturbed from their position, 
swim about as before until they meet with a 
similar situation. Their activity having ceased, 
they become permanently fixed, and are then 
about the size of the head of a very small pin. 
They appear to affix themselves to their rest¬ 
ing place by means of one of the little suckers 
with which their anterior extremity is fur¬ 
nished. 
The larva has now lost all power of loco¬ 
motion, and quickly undergoes further changes 
of form. The anterior extremity of the trunk 
is widened, and the prolongations of the in¬ 
ternal tunic quickly disappear. The central 
portion of the tail becomes at the same time 
empty, its contents being returned to the cen¬ 
tral yolk-mass of the body. The sac or internal 
tunic enclosing the yolk becomes much con¬ 
tracted and spherical ; lastly, the yellow matter, 
which was unequally divided, seems again to 
be rearranged. The tail, which during the 
early period of the existence of the larva per¬ 
formed so important a part, being the only 
instrument of locomotion, is now reduced to 
its gelatinous or tegumentary portion ; and 
this, after becoming more and more trans¬ 
parent, withers, and finally is detached, or falls 
away in shreds at a more advanced period of 
the growth of the animal. The trunk, on the 
other hand, is the seat of important and active 
changes. The tegumentary portion of the 
body is much widened, taking an oval out¬ 
line, and is visibly augmented in bulk. The 
interior tunic continues at first to lessen, and 
becomes quite spherical, and many large 
patches of a lighter yellow than the rest are 
apparent, one of which occupies the anterior, 
and two others the posterior portions of the 
tunic {fig. 784. »). 
M. Milne-Edwards further remarks, that 
the modifications already noticed ordinarily 
occupy the space of from ten to twelve 
hours ; and if the larvæ are again examined 
towards the end of the first day of their 
sedentary existence, further changes in the 
interior tunic may be observed. In a speci¬ 
men carefully watched by him, the follow¬ 
ing changes were noted. Instead of being 
spherical, the large yellow sac became oval, 
and its anterior part much thinned. It soon 
afterwards again became elongated, and a 
circular contraction divided it into two por¬ 
tions {fig. 784. e). The anterior portion, 
smaller and lighter coloured than the pos¬ 
terior, was rounded in front, and presented at 
that part a large annular patch of a deep yel¬ 
low, vaguely circumscribing a central paler 
portion. The posterior part was swollen and 
of a deeper yellow than the anterior, and quite 
behind there was observable a minute patch 
of a very clear yellow. This latter spot sub¬ 
sequently became the heart, and the annular 
spot on the other extremity of the body was 
developed into the thorax of the animal. The 
following day all these parts grew still more 
distinct. The anterior portion of the inter- 
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