IV] 



THE SECOND GRADE 



111 



any trace of communal existence, and only later was 

 one cell built up upon another into an individual of 

 a higher order. 



(a) (b) (c) (d) (e) 



Fig. 9. Diagram to show the probable evolution of the Catenata. 



(a) Gymnodinium pulvisculus, during its nutritive phase. 



(b) Blastodinium mycetoides. A nutritive cell remains per- 



manently attached to the host, and repeatedly divides 

 off reproductive cells from itself, 

 (c) (e) Haplozoon lineare. h, head-cell. 



(c) One-cell stage, resembling (a). 



(d) Two-cell stage, resembling (b) except that the two 



cells adhere to each other. 



(e) Adult, with reproductive cells about to be de- 



tached posteriorly. 



[Somatic nuclei black ; germinal nuclei white ; mixed nuclei 

 stippled.] (Modified from Dogiel.) 



To take parallel cases in a different grade of 

 individuality, the simpler Volvocidae closely resemble 



