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58
Saalænge dette Stadium i Udviklingen vedvarer, er endnu
ingen Madreporplade dannet; efterhaanden som Aabningen
i Hüden lukkes, udvikler Madreporpladen sig, og da nu
Stenkanalens ydre Ende vedbliver at være fastvoxet til
Huden, kan Madreporpladen ikke udvikle sig der, hvor den
ellers pleier at tindes hos Holothuriderne, men maa
frera-staa paa et andet Sted af Stenkanalen, som jo ogsaa virker
lig er Tilfældet hos Trochostoma.
Stenkanalens histologiske Bygning afviger ogsaa noget
fra Holothuridernes og synes at nærme sig mere
Asteri-dernes. forsaavidt den er kjendt, idet nemlig det indre
flimrende Lumen er omgivet af tæt tilsluttende Kalkrmge,
der dog ikke som hos Asteriderne sende Forlængelser ind
i selve Hulheden, hvorved denne bliver mere kompliceret,
men kun tjener som Støtte for det enkelte Lumen. Ogsaa
Madreporpladen danner et Slags Overgangsled imellem
Holothuridernes og Asteridernes, men synes at nærme sig
mest til de sidstes Madreporplade.
Vi have tidligere kun i Forbigaaende nævnt
Tentaklerne; vi skulle nu omtale dem noget nærmere. Som alle
Tentakler ere de hule cylindriske Rør, som paa deres
øverste frie Ender ere tredelte, Tab. VII, Fig. 4. Den
midterste Papille er den største og bredeste; Sidepapillerne
ere smalere og næsten lancetformige. Tentaklerne ere yderst
korte og rage knapt 2""" over Mundskiven. Deres ydre
Flade er beklædt med en yderst tynd, vandklar Cuticula,
under hvilken et enkelt Epithellag, bestaaende af
Cylinderceller, tindes. Indenfor dette sees en temmelig fast
Binde-vævshud, hvis Fibriller krydse hverandre i alle Retninger,
og i hvis intermediære Substants ere indleirede en Mængde
kugleformige Kalkkorn, der ligge meget tæt sammen uden
dog at berøre hverandre, og som tidligere ere beskrevne.
Til denne Bindevævshud fæster Muskelhuden sig, som
dannes af cirkulære og langsgaaende Fibre, hvilke tildels
ana-stomo8ere med hverandre.
Indenfor Længdemuskellaget iagttages et hyalint
Bindevæv, der er temmelig smalt, indeholder en stor Mængde
aflange kornede Celler (Sempers Slimceller) og enkelte
forgrenede Bindevævslegemer. Til dette Bindevævslag fæster
sig Tentakelkarret med sit flimrende Cylinderepithel, hvis
Cilier rage ind i Hulheden. Som tidligere nævnt ligge
disse yderst smaa Tentakler næsten skjulte i de før beskrevne
aflange Gruber, og kunne saaledes vanskeligen tjene som
Bevægelsesorganer, og heller ikke synes de paa Grund af
deres Lidenhed at kunne benyttes til Gribeorganer eller
til at føre Føden hen til Mundaabningen. En Funktion
maa de imidlertid have, men hvilken denne nu er. vide vi
ikke.
Kropshulheden er overalt beklædt med et flimrende
Epithel og er ved Vandkarringen forsaavidt afbrudt, som
denne danner Grændsen imellem Svælgsinus og den
egent-ige Kropshulhed. Idet Ringkanalen, som ovenfor angivet,
the water in which the animal is immersed. During
this stage of development the madreporic body does
not exist; it begins to form as the opening in the skin
gradually closes; but the outer extremity of the
sand-canal continuing to be connate with the integument, the
madreporic body cannot develop where it is commonly
observed in Holothurians; it must needs form on some
other part of the sand-canal, which indeed is the case with
Trochostoma.
The histological structure of the sand-canal differs
somewhat, too, from that in other Holothurians,
approximating apparently that in the Asteridce, — so far at least
is known, — the inner vibratile lumen being furnished
with tight-fitting calcareous rings, from which, however,
unlike those in the Asterida>. no prolations extend
into the cavity itself; they merely serve to strengthen the
particular lumen they encircle. Moreover, the madreporic
body constitutes a kind of transition link between the
Holothurians and the Asteridw, but would seem to bear
most resemblance to the madreporic body in the latter
family.
We have already occasionally mentioned the tentacles;
we will now proceed to describe them. They consist —
as do all tentacles — of hollow, cylindric tubes, which, at
their free extremities, are tripartite. Pl. VII. fig. 4. The
middle papilla is the largest and broadest; the lateral
papillæ are narrower and almost lanceolate. These
tentacles are exceedingly short, projecting at most 2m™ above
the oral disk. Their outer surface is invested with an
exceedingly thin transparent cuticle, under which extends
a single epithelial layer, consisting of cylindric cells.
Underneath the latter is seen a layer of comparatively firm
connective tissue, with fibrils interlacing in all directions, and
having, embedded in its intermediary substance, numbers
of globose calcareous granules, very closely arranged, but
not contiguous, similar to those already described. To this
layer of connective tissue is webbed the muscular integument*
consisting of circular and longitudinal fibres, some of which
anastomose one with the other.
Underneath the longitudinal muscular layer is seen a
layer of hyaline connective tissue, comparatively narrow, and
containing a large number of oval granulous cells
(Sempers mucous cells) and a few ramose corpuscles of connective
tissue. To this layer of connective tissue is attached the
tentacular vessel, with its vibratile cylindric epithelium,
the cilia of which project into the cavity. As
previously stated, these minute tentacles are almost
concealed in the elliptic-shaped cavities described above: and
hence they can hardly serve as locomotory organs, nor,
being so small, are they, it would seem, adapted for seizing
prey or conveying food to the mouth. Some function they
must however have, but what that function is, we are
unable to state.
The perivisceral cavity, invested over the whole
of its surface with vibratile epithelium, is intersected
by the water-vascular ring, inasmuch at least as the
latter constitutes the boundary between the pharyngeal
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