358 DR. N. ANNANDALE ON 



Colour. 



The colour of Hydra onentalis varies greatly in different individuals and even at 

 different times in the same individual ; it also varies as regards the whole race at differ- 

 ent seasons. 



When the polyps become common in the " tanks " in autumn, young four-rayed 

 individuals are colourless ; but adults and the five-rayed buds they produce are either 

 deep olive-green (often so dark that it is almost black) or orange-brown. In Calcutta the 

 latter shade is never very bright. Green individuals are at this season commoner than 

 brown ones, and it is rare for green ones and brown ones to be found on the same 

 plant. The buds of coloured individuals are of the same shade as their parents. As the 

 season progresses the number of brown individuals increases relatively, and others 

 appear which are neither green nor brown but of a peculiar shade which appears to be 

 as nearly as possible intermediate between the two formerly occurring. It is soon diffi- 

 cult to find an individual of the intense green prevalent in November, and though 

 orange-brown examples are less rare, they have a dull, faded appearance. As early as the 

 middle of February the greater majority of the specimens examined in 1906 were of the 

 intermediate shade, while some were already almost colourless. Polyps taken between 

 May and October show, at most, the faintest trace of colour. In captivity both brown 

 and green individuals assume the intermediate shade within a few days of capture, and 

 it is then impossible to distinguish between them. Finally, if they live long enough, 

 they become colourless. The few individuals which have produced eggs in my aquarium 

 have, however, retained a greenish colour ; while males, before producing spermaries, 

 frequently became of an opaque, milky white, owing to the presence of liquid globules 

 in their endoderm cells. 



The colouring matter of coloured individuals is not equally distributed all over the 

 body, but is most dense above the budding zone and immediately round the aboral pole. 

 It is sometimes almost absent from the tentacles. 



In this species colour is not due to the presence of symbiotic algae, but to minute, 

 amorphous particles in the cells l of the endoderm. Downing 2 points out that 

 the species of Hydra other than H. viridis change colour in accordance with their food. 

 That this is the case with Hydra onentalis I have little doubt ; but I have not been able 

 to prove the fact experimentally, because captive specimens lose instead of changing their 

 colour. 



This loss of colour is not always due to actual starvation. I believe it is generally 

 due to what may be almost called a form of indigestion. As I have already pointed out, 

 full digestion is a slow process and the faeces remaining after it are white andfloculent. 

 Under unfavourable conditions, however, such as obtain in an aquarium even when food 

 is abundant, full digestion does not take place. Food is only partially digested and the 

 faeces ejected from the mouth are brown and granular; should the prey have been of a 

 greenish colour, they may be tinged with green. That the origin of the colour of the 



1 Cf. T. J. Parker on Hydra fusca in Quart. Journ. Mia: Sci . XX, 1880, p. 222. 



2 Zool. Jahrb., Anat., XXI, 3, 1905, p. 381. 



