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79
ten Pteraster \ hos denne er ogsaa Aabninger i Ryghuden,
fuldkommen lig dem hos Hymenaster, og — hvad der ikke
tidligere har været observeret — at der hos Pteraster
lindes imellem de lange Naale, som udspænde
Hudbræmmen, en aflang Spalte, Fig. 18, a, der dækkes af en
membranes Klappe, Fig. 18, b, hvori en særegen, knæbøiet Pig
ligger indesluttet, Fig. 19, hvilken er forsynet med en
konkav Ledflade, Fig. 19, a, og et stærkt Muskelapparat,
som gjør, at Klappen med Lethed kan bevæges og
ligesom hos Hymenaster aabne og lukke Spalten. Disse
Spalter, der føre ind til Ryghulen ligesom Tilfældet er hos
Hymenaster, sees kun, naar Finnerne fjernes fra hverandre,
Fig. 18, c, og da kun ved Loupe, uden paa meget store
Exemplarer, hvor de kunne iagttages med blotte Øie.
Igjennem disse Spalter (semilunære Yentralaabninger hos
Hymenaster), Spiracula og Trakten, strømmer Søvandet ind i
Ryggens Hulrum, hvor det kan forblive, indtil det er
afbenyttet, idet samtlige Aabninger lukkes. Men imedens
hos Hymenaster den egentlige (indre) Ryghud, der
bedækker Kropshulheden, er overordentlig tynd og vel skikket
til at gjennemtrænges af Søvandet, er den indre
Ryghud hos Pteraster forholdsvis meget tyk og lader derfor
vanskeligt Søvandet passere, hvorfor der hos denne Slægt
Andes særegne hule Organer ved Grunden af Paxillerne,
der ere fæstede i Ryghuden, som aabne sig i
Krops-hulheden og ere homologe med Tentakelrørene hos
Aste-riderne. Foruden at Ryggens Hulrum tjener som
Respira-tionshule, saavel hos Hymenaster som Pteraster, tjener det
ogsaa som Udklækningshule for Æggene, der føres
gjennem en kort Kanal fra Æggestokkene ind i Hulrummet,
hvori Ungerne forblive under hele Embryonalperioden.
Fordøielses- og Generationsorganerne.
Maven er stærkt foldet, har ti korte Blindtarme, der
udgaa med hver sin korte Gren fra Mavesækken og lægge
sig tæt hen til den indvendige Rand af Armenes
Grunddel, Fig. 6, b. Fra Maven udgaar en kort men temmelig
tyk Rectum, der aabner sig paa det førbeskrevne Analfelt.
Den øverste Del af Maven tilligemed Rectum er bunden
til den indre Ryghud med mange, temmelig stærke Mesen-,
terialbaand. Ved Siden af Blindtarmene findes i det
tidligere beskrevne Rum, som dannes af 1ste Armpaxilles
Basaldel og Ambulacralpladerne, Kjønsorganerne, Fig. 6, c,
der bestaa af en Samling smaa Blærer i Form af
Drueklaser. De ere temmelig faste, ganske hvide, og fra dem
udgaa Smaagrene, som forene sig i en fælles
Udførselska-nal, Fig. 6. Denne, der er temmelig tyk og kort, munder
ud med en lille rund Aabning i Ryghulen paa hver Side
af Armenes Grunddel tæt ved Armvinklerne, Fig. 6, e.
Slægtskarakter.
Analaabningen central i Bunden af det store Hulrum,
ilar also occurs in the genus Pteraster. In that genus,
the apertures in the dorsal integument are exactly like
those in Hymenaster, but, what has not previously been
observed, is this; that in Pteraster, there is found between
the long needles expanding the integumental membrane, an
oblong fissure (Pl. XIII, fig. 18, b). In the flap, there
lies enclosed, a peculiar flexed spine (Pl. XIII, fig. 19),
and this is furnished with a concave articulatory surface
(Pl. XIII, 19, a), and a strong muscular apparatus which
causes the flap to be moved with ease, and like the
Hymenaster, to open and close the fissure. These fissures lead
into the dorsal chamber, as is also the case in Hymenaster,
and are only observable when the fins are separated from
each other (Pl. XIII, fig. 18, c); and only then, with the
assistance of a magnifier, except in very large specimens,
when they may be observed with the naked eye. Through
these fissures, (semilunar ventral apertures in Hymenaster)
the spiracles, and the funnel shaped prominence, the
water flows into the dorsal cavernous chamber, where it
is retained until it has served its purpose, when, all
the openings are closed. But, whilst, in Hymenaster, the
real (inner) dorsal integument which clothes the body
cavity is extremely thin, and well adapted for penetration by
water, the inner dorsal integument in Pteraster, is
proportionately very thick, and, therefore, permits the water to
pass with difficulty. In Pteraster therefore, there are found
special cavernous organs, situated at the base of the
pax-illæ and opening into the body cavity, and these are
homologous with the tentacular tubes in the Asteridæ. Besides
the dorsal cavity, in both Hymenaster and Pteraster,
serving as respiratory chamber, it serves, also, for hatching the
ova. These are conducted to it from the ovaries, through
a short duct, and the young starfish remain in it during
the whole of their embryonal period.
The organs of digestion and reproduction.
The stomach is formed by numerous laminating folds,
and has ten cæca, each with a short branch proceeding
from the stomach lining, and they lie close up to the
internal margin of the origin of the rays (Pl. XIII, fig. 6, b).
From the stomach, there proceeds a short, but rather thick
rectum, which opens into the previously described anal
area. The superior portion of the stomach, together with
the rectum, is connected to the inner dorsal integument,
by many rather strong mesenteric ligaments. Alongside
the cæca; in the space previously described as formed by
the basal portion of the first paxillæ of the ray and the
ambulacral plate; are seen the sexual organs (Pl. XIII,
fig. 6, c). These consist, of a collection of small vessicles
or blisters, resembling bunches of grapes. They are
tolerably consistent, and quite white. From them there
proceed several small branches, uniting in a common duct
(Pl. XIII, fig. 6). This duct is pretty thick and short,
and terminates in a small round opening into the
dorsal chamber, on each side of the origin of the rays, close
to the brachial angle (Pl. XIII, fig. 6, e).
Generic Character.
The anal aperture is central, and seated at the base
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