42 



THE INVERTEBRATA 



making the organism sensitive to light, which is of such importance 

 in its nutrition. 



The holophytic forms are usually capable of passing into a resting 

 phase, in which the flagella are withdrawn, the body rounded off, a 

 cyst or jelly case secreted, and the organism closely resembles a plant 

 cell. Division may take place in that condition, establishing a pseudo- 

 colonial stage known as the palmella, and from this there may be 

 built up a branched body (Fig. 38 D, D^) which simulates those of the 

 lower algae. Plant-like forms of this kind occur in every order of the 

 group. It is indeed impossible to define the Phytomastigina from the 

 Algae, and the members of this subclass are regarded both by 



Fig. 35- Fig. 36. 



Fig. 35. A section through a portion of the superficial tissues of Convoluta 

 roscoffensis, showing symbionts belonging to a species of Carteria (Chlamy- 

 domonadidae, Volvocina). From Keeble. ci. cilia of epidermis; epd. epi- 

 dermis; gr.c. "green cells" (symbionts); nu. nucleus of symbiont; pyr. 

 pyrenoid. 



Fig. 36. A free individual of the species of Carteria which is symbiotic in the 

 resting stage with Cotivoluta roscoffensis. From Keeble. chl. chloroplast; 

 e. eye-spot; nu. nucleus ; ^>'r. pyrenoid. 



botanists and by zoologists as coming within the scope of their 

 sciences. 



Many of the coloured species are liable to produce colourless in- 

 dividuals. This happens in two ways: the chromatophores may be- 

 come bleached owing to the animal living in darkness ; or the rate of 

 division of the chromatophores may lag behind that of the body, so 

 that eventually there are produced offspring for which there are no 

 chromatophores. These facts show how the colourless species may 

 have arisen. 



Members of various orders of the Phytomastigina (cryptomonads, a 

 chrysomonad, a chlamydomonad, and perhaps dinoflagellates) are 

 known to live in the resting stage as symbionts in holozoic organisms 

 (other protozoa, sponges, coelenterates, worms, etc.). Nearly all are 



