1 8 THE HELIOZOA 



seen at some periods than at others, and may even for a time dis- 

 appear without retraction of the pseudopodium. 



The nucleus is relatively large, with an obvious, doubly -con- 

 toured membrane. Within the membrane is a fine reticulum of 

 " linin " threads, on which are small particles of chromatin ; there 

 is generally also a single large extra-reticular mass of chromatin, 

 forming a karyosomatic " nucleolus." 



The vacuoles are of three kinds : non-contractile and contractile 

 vacuoles which do not contain food-particles, together with diges- 

 tive vacuoles which contain food. The non -contractile vacuoles 

 form a layer occupying the whole thickness of the ectoplasm ; they 

 contain a clear, colourless fluid, in which refringent granules, like 

 those found in the ectoplasm, may often be seen floating, the number 

 of such granules in a single vacuole being sometimes large. A 

 non-contractile vacuole, which contains many granules, sometimes 

 bursts, and the granules are scattered in the surrounding water. 

 There is generally only one contractile vacuole, which rhythmically 

 changes, enlarging slowly until its diameter may be about half that 

 of the body, and then suddenly collapsing ; the cycle of dilatation 

 and contraction is completed, at ordinary temperatures, in about 

 one minute (40-100 seconds, Penard [14]). The function of the 

 contractile vacuole is as obscure in this as in other cases. Most 

 observers believe that the fluid, collected during dilatation, is 

 expelled from the body during contraction of the vacuole, so that 

 the whole process is excretory in nature ; but while it is difficult to- 

 watch an Adinophrys without sharing this opinion, it is equally 

 difficult to demonstrate its truth. The contraction takes place so 

 quickly that it is impossible to be sure whether a rupture of the body- 

 wall occurs or not ; and all attempts to show that the collapse of 

 the vacuole is accompanied by a disturbance in the surrounding 

 water, such as would result from the forcible expulsion of its con- 

 tents, have hitherto failed. 



Food-vacuoles are formed in the blunt processes of the ectoplasm 

 already described. When fully formed they contain a clear fluid, 

 surrounding the ingested food-mass, which doubtless contains some 

 solvent in solution, analogous to those demonstrated in the similar 

 vacuoles of amoebae and of ciliata. Formed immediately beneath 

 the surface of the body, the food -vacuole remains throughout 

 its whole existence in the ectoplasm, where the processes of diges- 

 tion and absorption are completed ; a vacuole with a large food- 

 mass may, however, travel into the deeper parts of the ectoplasm. 

 After digestion is completed the residue of the food-mass remains 

 in the vacuole for some time, being ultimately discharged by the 

 bursting of the vacuole at some part of the surface of the body. 



The food consists of living organisms, animals and plants. 

 Smaller prey is seized by the blunt ingestive processes alone, with- 



