AN INTRODUCTION TO ZOOLOGY. 



applied to similar light given off by certain vegetable 

 organisms, especially fungi, and by some flowers. 



Some of the Protozoa show phosphorescence, and 

 also many small forms of the Hydrozoa, and these 

 give a vagne general phosphorescence to the breaking 

 foam of the sea, which in our climate is usually best 

 seen on a warm summer evening, when the air is some- 

 what oppressive, as, for instance, in thundery weather, 

 although it may occur on mild days, even towards 

 the winter. One of the best instances among the 

 Protozoa is 'Sodihica (see p. 138). Among the Ccelen- 

 terata the larger forms are also often very conspicuous, 

 such as the Girdle of Venus, which is found in the 

 Mediterranean, and the large globular jelly-fish of the 

 Atlantic Seas, which are often several feet across. 

 There are also instances of phosphorescence among 

 nearly every other group of animals, including worms 

 and Crustacea. Some molluscs (Pholas) are luminous, 

 and several of the ascidians are brilliantly so, especi- 

 ally Pyrosoma and Saljja. It is to be supposed that 

 the use of phosphorescence to marine animals is to 

 light up their way in the sea, and prevent collision 

 with one another. In this way it would be of great 

 use to the animal bearing the light, so that we can 

 understand why it should exist in the case of some 

 animals which have not eyes themselves, and even of 

 some which have become blind through long residence 

 in the depths of the sea. Numbers of terrestrial insects 

 are luminous, and with them the chief use of the liwht 

 seems to be as a means of communication, especially 



