I.—PHYSIOLOGY. 205 
nitrogenous substances in the dark and by light carbon-containing 
substances. Organisms harbouring alge exhibit naturally a positive 
heliotropism. 
Symbiotic algz are not usually transmitted hereditarily, each host- 
generation being usually infected afresh by alge, encountered about the 
host, which may be either free-living or possess a free-living stage in 
their development. Exceptions occur, however, where Protozoa multiply 
by division and the alge pass directly (as it were hereditarily) to 
succeeding generations. There are also cases of hereditary transmission 
in hosts that undergo sexual multiplication (as in Hydra viridis), the 
zoochlorella penetrating the egg on escaping from the host’s endodermal 
cells after the manner of starch granules or other food reserve sub- 
stances (vy. supra). From the circumstance that in most cases symbiotic 
alge are not transmitted hereditarily, we may explain the occasional 
occurrence of alga-free individuals in a species usually harbouring the 
symbionts. 
Studies conducted on TurBELLARIA are of special interest: These 
animals may contain either green or yellow symbionts, and, as in 
Protozoa, some allied species harbour the symbionts and others do not. 
The eggs of Turbellaria are symbiont-free, each generation becoming 
infected afresh, the symbiont either entering the host’s mouth and 
“remaining there, traversing the intestinal wall, or entering by the genital 
pore, according to the particular host-species it affects. 
The best-known example of symbiosis in Turbellaria is found in 
Convoluta roscoffensis, a species that has been well studied by Keeble 
and Gamble (1903-7). Its larve are colourless and infection occurs 
after hatching. Colourless larve are obtainabie by transferring freshly 
hatched examples immediately to filtered sea-water. The cocoon, on 
the day following its deposition, is already invaded by many alge having 
a very different structure from those found in Convoluta; they possess 
four flagella and have been referred by Keeble and Gamble to the genus 
Carteria (allied to Chlamydomonas). The alge within the host possess 
a special structure, their contour is very irregular, they have no cellulose 
_ wall, the green colouring matter is unevenly distributed, being confined 
to chromatophore bodies surrounding the pyrenoid body, the nucleus is 
_ eccentric, and a number of examples are found with degenerating nuclei. 
Naturally all attempts to cultivate these obviously degenerating alge 
have failed. 
_The physiological relations existing between Turbellaria and alge 
differ according to the species. Thus in Vortes viridis symbiosis is not 
hecessary, in Convoluta it is necessary for both partners. Mature 
Convoluta are never found devoid of alge in nature. The young larva 
can only feed itself for a week; as it grows older it becomes infected 
progressively with alge and ceases to nourish itself otherwise than 
upon the products of its contained symbionts. Finally, having reached 
an advanced age, the animal becomes capable of digesting the alge, as 
does Convoluta paradoza under unfavourable conditions of life. Keeble 
and Gamble define four periods in the life of Convoluta, which they term 
respectively hetero-, mixo-, holo-, and auto-trophic, wherein the animal 
