192 COMPARATIVE ANATOMY 



only active in sunlight, this raises the presumption that the 

 organism is able to obtain food in some other \vay. Experi- 

 ments have shown that if they are kept in carefully filtered 

 water Euglense will not flourish, even in the sunlight, but 

 that they will recover on the addition of small quantities of 

 albuminous matter to the water. Hence it is concluded that 

 they absorb organic substances in solution by the whole 

 surface of the body, and it is supposed that during the daylight 

 they form paramylum, and during the night they absorb 

 organic food in solution. It must be confessed, however, 

 that there is no very satisfactory evidence on the subject of 

 the nutrition of Euglena, except the well-established fact that 

 it decomposes carbonic acid and forms paramylum. 



A noticeable feature in Euglena is the presence of a speck 

 of bright red colour immediately behind the anterior end of 

 the body and close to the inner end of the conical depression 

 called the gullet. This spot is the stigma. It has a some- 

 what quadrangular outline, lies close against a spherical clear 

 cavity which will be described in connection with the con- 

 tractile vacuoles, and consists of a meshwork of plasma, con- 

 taining a number of minute red granules composed of a 

 substance called haematoclirome. This red pigment is common 

 in many flagellates, and it is particularly abundant in a red 

 species known as Euglena sanguinea. It becomes blue-black 

 with iodine, is soluble in ether or alcohol, but is not affected 

 by ammonia or acetic acid. According to a recent author the 

 stigma is covered anteriorly by one or more strongly refracting 

 granules of paramylum, and the whole structure serves as a 

 rudimentary organ of vision, the paramylum granules forming 

 the lenses, the stigma the percipient elements. It is at the 

 best doubtful whether the stigma is specially sensitive to light, 

 but it is significant that the haematochrome disappears in 

 individuals kept in the dark, and that it has the same reactions 

 as the pigments found in the eyes of many higher animals. 



The clear vesicular space against which the stigma appears 

 to be flattened has often been confused with the contractile 

 vacuole. It is not, however, contractile, but serves as a 

 chamber or reservoir into which several minute contractile 

 vacuoles discharge their contents. These minute contractile 

 vacuoles have much the same structure as in Amoeba; there 

 may be one only, or several, as in Euglena viridis, and when 



