CORALS. 
5*3 
have remained at a lower stage of development, are smaller, and have neither 
tentacles nor reproductive organs. These zooids appear to perform only one 
function, namely, to pump water through the body of the stock. In this important 
work the higher individuals no doubt assist, as indeed in most alcyonid-stocks 
they alone must do the work, there being no such specialised pumping-polyps. 
In addition to the small, isolated calcareous particles already mentioned 
within the bodies of the individual polyps, sea-pens have a further support in the 
form of a calcified and often flexible axis, entirely concealed in the stock and 
pointed at the two ends. The accompanying illustration represents Pteroides 
spinosa, in which the polyp-bearing leaves are supported 
by a number of calcareous rays which project at the 
edges as spines. 
The best known of the sea-pens is the phosphorescent 
Pennatula pliosphorea of the Mediterranean and the 
Atlantic. In this form the capacity for giving light is 
not possessed by the whole surface of the stock, but 
only by eight band-like organs on the polyps themselves, 
the upper ends of which surround the mouth like 
papillm, while their lower parts run down over the 
stomach. These bands are filled with cells containing 
fatty spherules, to which the phosphorescence is con¬ 
fined. The fact that these luminous bodies have been 
found in all parts of the stock is explained by the 
liability of the bands to be injured, the least pressure 
causing their contents to escape. Specimens which have 
been roughly treated when captured and have strongly 
contracted, as also those which have been kept for any 
length of time in small vessels and have become dropsical, 
are incapable of giving light. The phenomenon is only 
observed in freshly-caught and little-disturbed animals 
Very slight irritation, such as is produced by tapping on 
the glass of the aquarium, is enough to call forth flashes. 
If the animal is taken in the hand, either in or out of water, still brighter luminous 
spots and streaks are seen. Repeated careful experiments have revealed the fact that 
the streams of light follow regular courses. There are two kinds of streams; the 
one connected with the polyps proper, and visible on the dorsal side of the feathers, 
and the other connected with the zooids, and appearing on the lower side. The 
two streams appear at the same time, as a rule, but either the one or the other 
may, without any apparent reason, arise independently. It can be shown that the 
direction taken by the streams depends on the part to which irritation is applied. 
The higher forms of the sea-pens, or those which actually resemble feathers, are 
not found in deep water, none being recorded to exist below six hundred fathoms. 
Deep - sea forms have, however, been found; these being related to Umbellula 
grcenlandica, which has long been well known. As early as the middle of last 
century, when the presence of animal life at great depths was quite unknown, two 
specimens were brought up from a depth of two hundred and forty fathoms, 
vol. vi .—33 
V-S, h /> 
a sea-pen, Pteroides spinosa 
(J nat. size). 
a, A polyp (somewhat magnified). 
