26 GENERAL HISTORY OF THE rNFUSOEIA. 



united, and their mode of combination, differ in^ the different genera and 

 species. 



In Scenedesmus the cells are arranged (I. 37 to 43) in single linear series, 

 side by side, united by a mucous hj^aline matrix, which is less abundant 

 than in Pediastrum. Two, mostly four, and less frequently eight cells are 

 concatenated ; and, as a rule, the line of union extends the entire length. Ex- 

 ceptions occur, owing to the junction-surfaces being less extensive, in the form 

 of chains of cells having a zigzag border, every alternate cell being depressed 

 below the normal plane, or in that of an obhque chain, having each member 

 in succession depressed beneath the preceding. Sometimes two rows of eight 

 cells each lie side by side (I. 38), so that the one dovetails into the other by 

 the alternate elevation and depression of their component cells : this may 

 happen in the whole extent of the two coherent chains, or in a portion 

 only of their length at one or other extremity ; or one chain may be broken 

 into two segments, each dovetailed to the other chain at opposite ends so as 

 to leave an unoccupied central space. The alternation of the cells in fronds 

 composed of two rows, is the result, according to Mr. Ealfs, of the obhque 

 manner of division. In the genus Pediastrum the fronds are generally com- 

 posed of a larger number of cells than in Scenedesmus, disposed in the same 

 plane according to a definite and usually concentric arrangement, and foiming 

 compound steUate fronds (I. 52, 53, 62, 66, 67), whence the term Micras- 

 terias (httle star-like beings) invented by Ehrenberg, and also the second half 

 of the generally adopted term {Pedi)-astrum. 



To distinguish species, Ehrenberg chiefly employed the number of the 

 cells in a frond — both the entire number and that of each concentric circle, 

 together with the number of circles. Succeeding naturahsts, however, have 

 pointed out that the number of cells in the same species is subject to con- 

 siderable variation. TmT^)in detected the true law determining their number, 

 and Nageh fui^ther illustrated and enforced it. The latter wiites that 

 {Einzell Alg. p. 92) '' the cells are united 2, 4, 8, 16, 32, or 64 together 

 in a frond. These numbers are always constant in young fronds without 

 exception. In older specimens one or more cells may be lost, and the frond 

 become therefore apparently irregular. These cells do not spontaneously 

 detach themselves from the rest, but die, and are partially or entirely 

 dissipated, as a consequence of injury from some external cause, probably in 

 most cases by small aquatic animals. They occur in all stages of destruc- 

 tion, and when entirely vanished, the vacant space indicates their fonner 

 position. The cells are aggregated together in a single plane, which possesses 

 mostly a circular or somewhat rounded outline ; but in the disposition of the 

 cells there is considerable variety. In the case of 4 cells they are either all 

 in opposition (II. 27), or only 2 in the centre ; with 8 cells, one usually 

 lies in the middle, and the other 7 surround it in a circular manner 

 (I. 52); less commonly, 2 are central and 6 peripheral (I. 62); rarely, 

 one occupies the centre with 6 around it in a circle, whilst the remaining 

 or eighth cell is placed on the perii^heiy ; and stiU rarer, the disposition is 

 quite irregular. Where 16 cells are combined, the rule is that there is one 

 in the centre surrounded by an inner circle of 5 and an outer circle of 10. 

 At times, 4, 5, or 6 internal cells are encii^cled by 12, 11, or 10 outer 

 ones (I. 53, 66, 67), whereby a double ring is produced: more rarely 

 the arrangement is completely irregular. Again, 32 cells are mostly 

 so placed that one central cell has around it 3 circles, the innermost of 

 5, the middle of 10, and the outer of 16 cells ; less frequently, the 3 

 circles are respectively composed of 5, 11, and 15, or of 6, 10, and 16; 

 occasionally 5 internal cells have 2 outer scries, one of 11, the other of 16 



