THALLOGEN^. 719 



brownish colour, valvular, and invested by a siliceous mem- 

 brane. Examples : — Diatoma, NaAdcula. 2. Desmidieoe 

 {fig. 1096). Found only in fresh water, of a green colour, 

 continuous, and not invested by a siliceous membrane. Ex- 

 amples : — Closterium, Dcsmidium. 

 Sub-order 5. Volvocinea (fig. 1097). Henfrey diagnoses 

 them as follows : — " Microscopic bodies swimming in fresh 

 water by the aid of cilia arranged in 

 pairs upon the surface of a common Fig. 1097. 



semi-gelatinous envelope, the pairs 

 of ciha each belonging to a green 

 corpuscle resembling the zoospore 

 of a confervoid, imbedded in the 

 periphery of the common envelope. 

 Reproduction by the development of 

 each corpuscle into a new colony, 

 the whole being set free by the solu- 

 tion of the parent envelope or by 

 conversion of the corpuscles into 

 encysted resting- spores like those of f^, 1097. The Revolving 

 Confervae." Examples: — Volvox, y oivox {Voivox giobator), 

 Gonium. The members of this T^/^o^ter surface is cm- 

 group are frequently regarded as 



Infusorial Animalcules, but in all their essential characters 

 they closely resemble the Confervoideae. 

 Distribution, ^c. — Algas are more or less distributed through- 

 out the globe, growing in salt or fresh water, or in moist situa- 

 tions. Some species are found in the boihng springs of Iceland, 

 &c. ; others, occur in mineral springs, and some in chemical 

 solutions. The waters of whatever temperature have their own 

 peculiar forms. It is impossible to estimate with any degree 

 of accuracy the number of species of Algae, but they may be 

 roughly guessed at 2500, 



Properties and Uses. — Several species are employed for food 

 in different parts of the world; as, Laminaria saccharina, digi- 

 tata, potatorum, Sfc. ; Alaria esculenta, Durvillcea utilis, Sargas- 

 sum species, Iridoea edulis, Chondrus crispus and mamillosus, 

 Gelidium comeum, 8fc., Gigartina speciosa, Laurencia papillosa, 

 Sfc, Gracilaria lichenoides and other Gracilarias, Rhodymenia 

 palmata, Porphyra vulgaris and laciniata, Ulva latissima, com- 

 pressa, Sfc; Nostoc edule, arcticum; and many others. The nu- 

 tritious properties of the above are due to the presence of starch, 

 sugary matter (mannite), mucilage, and albumen, J^. Payen 

 has recently discovered a new principle in Gelidium comeum 

 {Algue de Java), and some other Algae, to which he has given 

 the name of Gelose. To this substance also the nutritious pro- 

 perties of Algse are, to a great extent, due. According to Payen, 

 1 part of gelose dissolved in 500 parts of boiling water, will 

 3 A 



