224 



FOUNDERS OF OCEANOGRAPHY 



fully known, but the two ruby lights owe their colour to a 

 screen of red chromatophores in the skin (Fig. 13). 



TuNiCATA (Ascidians). — Only one, very remarkable, case 

 need be discussed in this group — that of Pyrosoma. This is a 

 large, free-swimming colony in the form of a hollow cylinder 



Fig. 13. — Sketch of Deep-sea Luminous Cuttle-fish with numerous 



light-organs. 



with one end closed (Fig 14). The walls of the cyHnder 

 are formed of the ascidiozooids, or members of the colony, 

 placed closely side by side, with their mouths on the outer 

 surface. Each ascidiozooid has two photogenous glands 

 placed one on each side of the anterior end of the body a 

 little behind the mouth, and therefore close to the outer 

 surface of the colony. Each gland consists of a mass of 







Fig. 14. — Small Colony of Pyrosoma^ natural size. 



granular cells surrounded by a blood sinus. The light is 

 described as red in some cases and blue in others, and as a 

 colony only a few inches in length may have several thou- 

 sands of these sparkling points, the volume of light emitted 



