282 BRITISH SEA-WEEDS. 



gelatinous substance of the threads are characters suf- 

 ficient to justify the removal made by Dr. Gray in Tris 

 ' Handbook of Water-weeds/ and which I have adopted. 

 This plant is a summer annual, and may be found on 

 many parts of our coast. 



Order XXIV. OSCILLATORIACE.E. 



Green (rarely Olive-broivn, Blue, or Purple), marine or 

 freshwater Algce, composed of simple or slightly -branched 

 threads, each of ivhich consists of an annular, medullary 

 chord, of very short cells, enclosed in a one-celled, mem- 

 branous sheath. 



Only two or three genera of the plants belonging to 

 this Order are marine. The remainder inhabit water of 

 all degrees of quality and temperature, or grow in places 

 which are merely damp. Among them are to be found 

 the tenants of the Geysers of Iceland, and of the vats 

 of poisonous chemicals, etc., to which I have already re- 

 ferred in the second chapter of this work. Certain kinds 

 exhibit peculiar motions, similar to those of the lower 

 forms of animal life, and are capable of progression and 

 retrogression, and of ascent and descent in water. 

 Speaking of the Oscillatorice, Dr. Harvey writes, " These 

 movements are of three kinds : first there is the oscilla- 

 ting movement ; one end of the thread remaining nearly 

 at rest, while the other sways from side to side, some- 

 times describing nearly a quarter of a circle in a single 

 swing. Secondly, the tip of the filaments has a minute 

 movement, bending from side to side like the head of a 

 worm ; and, thirdly, there is an onward movement, pro- 

 bably the result of the two former. It is this latter 



