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 phosphorea of the Mediterranean and the 

 Atlantic. In this form the capacity for giving light s 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 

 papillae, 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 ba 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 



a sea-pen, Pteroides spinosa 

 (I nat. size). 



a, A polyp (somewhat magnified). 



