70 COCCOPHYCE.E. 



Gonium pectorale. Mull. Vermium,p. 60. 



Coenobium flattened, quadrangular, composed of 16 green 

 cells, furnished with vibratile cilia. 



SIZE. Coenobium, from -05 mm. Cells from '01 X '007 mm. 



Babh. Alg. Ear. iii. 99. Mull. Anim. Inf. t. 16, f. 9-11. 

 Ehrb. Infus. 56, t. 3, f. 1. Dujard. Zoophy. p. 318. Focke 

 Studien, 30, t. 4, f. 7, 8. Fresen, Abhand. der Senck, p. 191, 

 t. 8. Cohn Nova Acta, xxiv. p. 169, t. 18, f. 9-27. Prit- 

 chard Infus. p. 518. 



In stagnant water. 



The fullest account of Gonium is that by Cohn, published many years 

 ago, but nothing has been added to its history since. The following is a 

 summary of his observations : 



Each family is invested with a colourless mucous sheath which is diffi- 

 cult to be seen without adding some colouring matter to the water, as 

 there appears to be no tegument. Seen from above, it is a quadrilateral 

 tablet with rounded corners, or from the side elliptical. The primordial 

 cells are sixteen in number, four occupying the centre disposed as a 

 square, and three on each side external to these. The central cell of the 

 three on each side is seta little nearer towards the centre. The cells are 

 somewhat polygonal, the four central being hexagonal, and the twelve 

 external pentagonal. When young the angles can scarcely be dis- 

 tinguished. This regular polygonal form indicates that each cell is sur- 

 rounded by a firm membrane, which retains them in a fixed form. The 

 investing membrane may also be detected at the angles of the cells, from 

 each of which it is extended in a short tubular process, which is quite 

 colourless. These pi'ocesses are joined to those of contiguous cells, so 

 as to link them all together. These processes are not visible in imma- 

 ture families, being subsequently developed. 



In other points the organization, of the cells resembles Chlamydomonas. 

 Their contents consist of protoplasm, coloured by chlorophyll, when old 

 containing numerous corpuscles, a central darker corpuscle, and often 

 several vacuoles. Each cell is furnished with two cilia, which proceed 

 from the protoplasm, through perforations in the cell wall. 



The movements of the coenobium resemble those of Stephanosphcera ; 

 it revolves on its short axis, so that its polar aspect is that of a rotating 

 surface, whilst its equatorial is linear. 



In the development by subdivision only the cell-contents are con- 

 cerned. The fission takes place by four successive stages, or genera- 

 tions, in each of which bisection of the cells occurs, so that ultimately 

 each primordial cell is subdivided into sixteen portions. When the sub- 

 division is completed by the construction of these sixteen small cells, 

 they are seen to occupy the same position and arrangement in the 

 mother-cell as in the parent coenobium. 



The primordial cells of the newly formed coenobia appear unconnected 

 with each other. The movement of the pai'ent ccenobinm continues 

 until the last stage of fission is completed, when it ceases, and the 

 young daughter coenobia commence a movement within the parent cell, 

 sometimes appearing as a disc, and at others as a line, according as the 

 surface or the edge is turned towards the spectator. 



At length the mother-cell ruptures, and the young colony escapes into 

 the water, moves about freely, and starts on an independent existence. 

 Supposing that a young Gonium after twenty-four hours is capable of 

 development by fission, it follows that, supposing the conditions favour- 



