Tank No. 21. 
19 
Fig. 13. Pennatula phos- 
pliorea in extended condi¬ 
tion, V 2 nat. size. 
Of these tree-like corals 
(tank 21) we would mention 
first the Sea-finger ("dead- 
men s-fingers”), Alcy onium 
(Fig. 12), the skeleton of 
which does not form a united 
framework, but consists of 
numerous loose calcareous 
spicules; so that the animals 
are able to swell themselves 
out by taking up a large 
amount of sea-water. 
The Sea-pen, Pennatula 
(Fig. 13), can also swell 
itself out at pleasure by- 
taking in sea-water. When 
not thus expanded the body 
is flabby, and the animal to 
all appearance dead; but 
when it has taken up sea¬ 
water it becomes beauti¬ 
fully transparent and erect, 
and the leaf-like lappets 
are studded along their up¬ 
per edge with delicate polypes like transparent 
flowers. The Sea-pen can move about freely 
and burrow deep into the sand with its stalk. 
At night it phosphoresces, when roughly handled, 
with a greenish light. 
The family of Corticate Corals are well 
represented in the Aquarium (tank 21). To 
begin with we have as Horny Corals the Black 
Coral, Antipathes (Fig. 14), and the Sea-fan, 
Gorgonia (Fig. 15),. possessing flexible skeletons 
, which consist of a horny substance. The branches 
of Gorgonia all lie in one plane, and when the 
polypes protrude their bodies and expand their 
tentacles, they project like small feathery flowers 
on either side of each branch. In the Bay of 
Naples there exist white, yellow, and red Sea- 
fans, thelatterbeingthemostnumerous. Antipathes 
has a glossy black skeleton which attains in 
large specimens the thickness of a finger; it is 
used, although rarely, as the so-called "black coral” 
for ornamental purposes. The skeleton of the 
White Coral, Isis (Fig. 16), consists of alter¬ 
nating pieces of horny and calcareous matter; in 
the Red Coral which belongs to the same family, 
the skeleton is entirely carbonate of lime (chalk). 
Fig. 14. Antipathes larix 
attached to a stone, 
nat. size.. 
V2 
