48 LIFE IN THE SEA [CH. 



much less in May and June and are succeeded by 

 others which we may regard as summer forms. In 

 this part of the sea the summer diatoms belong to 

 the genus Rhizosolenia and they are always accom- 

 panied by enormous numbers of the pelagic alga 

 Halosphera, a species which forms little mucilaginous 

 capsules which are easily visible to the naked eye. 

 During the month of June, and sometimes part of 

 July, the sea off the coasts of Xorth Wales swarms 

 with these organisms, and no biological phenomenon 

 in the Irish Sea is more remarkable than this in- 

 vasion. The diatoms and the other algae can easily be 

 seen with the naked eye in a small quantity of water 

 from the beach even so little as will lie in an empty 

 mussel shell ; and the nets of the fishermen are often 

 covered with a slimy mass produced by the bursting 

 of the capsules of Halosphera. Both species are intro- 

 duced by the inflowing Gulf Stream drift which is 

 strongest along the west coast of England and Wales, 

 and they reproduce when the temperature ' of the 

 water is rising rapidly. Some time in the month of 

 July both species disappear almost entirely and then 

 the sea is invaded by shoals of jelly-fishes, belonging 

 chiefly to the species Aurelia and Rhizostoma, but by 

 the end of the summer these too become much less 

 in number and during the autumn they sink to the 

 bottom and die. Shortly after they have attained 

 their maximum of abundance the sea becomes 



