GREEN ALGvE 135 



Perforating AlgSB. Several Myxophycese and a few 

 Chlorophyceae live in the shells of molluscs, in the 

 skeleton of corals, or in calcareous algae, where they 

 bring about bluish, green, or violet spots. These 

 s< perforating algae," as they are called, on account of 

 their power to bore into calcareous tissues, are known 

 from the temperate and tropical seas. By their in- 

 cessant labour they destroy many shells, corals, and 

 calcareous algae, setting free carbonate of lime. 



The Chlorophyeeae, or green algae, abound in the 

 littoral, and are also found in the sublittoral region. 

 Their usual way of reproduction is by spores, which 

 move by the aid of cilia. Some of these spores conjug- 

 ate, others germinate without conjugation. Of Cau- 

 lerpa no reproductive organs are known. Many forma- 

 tions of green algae are known from the temperate and 

 tropical seas. The filamentous Urospora, Chaeto- 

 morpha (Fig. 46), Cladophora (Fig. 48), the mem- 

 branelike Monostroma and Ulva (Fig. 49), the tube- 

 like Enteromorpha, prevail in the temperate seas, 

 while the large group of the Siphoneae occurs chiefly 

 in the tropics or in warmer temperate seas. The 

 Siphoneae attain a very high outer differentiation, but 

 each member of this group, however complicated the 

 structure of its frond may be, consists of only one 

 multinucleated cell ; therefore they are called Siphoneae 

 from their tubular nature. Many Siphoneae Hali- 

 meda (Figs. 50, 51), Neomeris, Acetabularia (Fig. 53), 

 Udothea (Fig. 52), Bornetella (Fig. 55), Penicillus, etc. 

 are encrusted with carbonate of lime, and to this 

 encrustation Halimeda owes its importance as a reef- 

 forming organism. The Caulerpae (Figs. 54, 56) cover 

 the coral reefs with numerous species of multifarious 

 aspect, or live in a depth of several fathoms. Outside 

 the tropics only a few representatives of this family 



