LITHAMCEBA DISCUS. 485 



represented as seen in the living condition, quiescent. It 

 consists of a discoid mass of protoplasm the TTrth of an inch 

 iu diameter. 



The concretions. — The periphery of the disc is clear and 

 colourless, towards the centre a dark greyish appearance is 

 observed, owing to the large number of rounded concretions 

 of a highly refringent substance which are embedded in 

 the protoplasm. Most of these concretions are of minute 

 size, with a tendency to a reniform shape. Two much larger 

 concretions are seen, the larger of which measured the ^i-oth 

 inch in length. The substance of which these concretions 

 are formed was not determined. It resists the action of dilute 

 acetic acid and of dilute caustic potash, but is dissolved by 

 strong hydrochloric acid. 



In fig. 8 one of these concretions is represented from 

 another specimen isolated. 



The nucleus. — A single nucleus (w), of large size, measuring 

 -g-J-oth inch in longest diameter is present. It has an irregular 

 block-like form and a very obvious and definite structure. 

 It is enclosed in a well-differentiated membrane, which can 

 be separated from it by the action of reagents (fig. 5). Its 

 substance appears to be built up by a number of minute, 

 closely-sef granules, which are angular and set side by side 

 in a cementing substance. There is no specialised nucleolus, 

 nor are nucleolar fibrillae to be observed. 



Food matters, — Besides the concretions and the nucleus 

 the protoplasm contains a quantity of food debris {ff), con- 

 sisting of a frustule of the Diatom Navicula, and the carapace 

 of a Rotifer and other matters. 



Contractile vacuole. — The centre of the disc is occupied by 

 a very large vacuole (cw), containing a clear liquid, and 

 having, both above and below, excessively thin walls. The 

 vacuole measures -3-io^th inch in diameter. Continued ob- 

 servation showed this vacuole to be contractile, and that its 

 contents are discharged periodically to the exterior. 



Vacuolar structure of the protoplasm. — In focussing the 

 upper wall of the vacuole I first became aware of the ex- 

 cessively fine reticulate or vacuolar structure which charac- 

 terises the protoplasm of the whole body. This differentiation 

 of the protoplasm can be detected all round the margin of 

 the disc also, and, in fact, wherever the protoplasm is 

 sufficiently free from concretions or food matter to allow of 

 proper illumination and inspection. This vacuolar structure, 

 as seen under a No. 10 immersion objective after treatment 

 of a specimen of Lithamceba with osmic acid, followed by 

 picro-carmine, is represented in fig. 4. 



