The minute
structure of these bodies shows an extremely delicate tracery
of fine tubes, sometimes empty, sometimes filled with loose
calcareous matter dyed with peroxide of iron."--(Sir Wyville
Thomson.) Many of the Chalk sponges, originally calcareous, have
been converted into flint subsequently; but the Ventriculites
are really composed of this substance, and are therefore genuine
"Siliceous Sponges," like the existing Venus's Flower-Basket
(_Euplectella_). Like the latter, the skeleton was doubtless
originally composed, in the young state, of disconnected six-rayed
spicules, which ultimately become fixed together to constitute
a continuous frame-work. The sea-water, as in the recent forms,
must have been admitted to the interior of the Sponge by numerous
apertures on its exterior, subsequently escaping by a single
large opening at its summit.
[Illustration: Fig. 187--_Kotalia Boueana_.]
[Illustration: Fig. 188.--_Siphonia ficus. Upper Greensand. Europe.]
[Illustration: Fig. 189.--_Ventriculites simplex_. White Chalk.
Britain.]
Amongst the _Coelenterates_, the "Hydroid Zoophytes" are represented
by a species of the encrusting genus _Hydractinia_, the horny
polypary of which is so commonly found at the present day adhering
to the exterior of shells. The occurrence of this genus is of
interest, because it is the first known instance in the entire
geological series of the occurrence of an unquestionable Hydroid of
a modern type, though many of the existing forms of these animals
possess structures which are perfectly fitted for preservation in
the fossil condition.
Pages:
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401