THE NUCLEUS OR ENDOPLAST IN THE INIUSORIA. 



by a definite circle of endoplasm, but has rays, or tubular passages, tapering outwards around it 

 (Fig. 2, cv). The passages are numerous, and may be seen to ramify at their extreme ends, and they 

 are weak spots in the cortical layer over the more fluid endoplasm, extending far and wide from the 

 vesicle. These passages transmit light more readily than the protoplasm in which they are placed, 

 and it therefore occurs that, as water fills them, and they increase in diameter and length, they are 

 nearly as light-transmitting as the main space with which they are continuous. They become largest 

 just before the contraction of the vesicle, and they sometimes do not disappear until after its 

 contraction. It is evident that the watery contents of the passages are pressed upon by the con- 

 traction of the surrounding protoplasm, and that this water and that of the space penetrate, 

 during contraction, into this environing substance. Movement may be noticed under the outer 

 tissue, here and there, within and along the lines of the passages. More or less defined com- 

 munications exist between the outside water and the contractile vesicles through the ectoplasm, 

 and the vesicle receives pure water from without, and collects 

 and expels the impure water from within the animal. It is 

 evident that the function of the contractile vesicle is of great 

 importance to the animal, and it may relate to the elimination 

 or removal of certain soluble matters resembling the urinary 

 secretions. It may also relate to an internal circulation of water. 



The rhythm of the dilatation and contraction is very 

 remarkable, and Saville Kent states that "the time occupied 

 between the consecutive pulsations of this organ is found, 

 under normal conditions, to present a constant average among 

 individuals of the same species, varying from a few seconds only 

 in certain forms, to over sixty or even one hundred seconds in 

 other types." 



The nucleus or endoplast with its contents resembles, in 

 some Infusoria, that of the simplest vegetable cell. In its 

 simplest form, noticed in some of the Flagellata, the endoplast is 

 more or less spheroidal, and may or may not contain a nucleolus 

 or endoplastule. Saville Kent has given an admirable resume 

 of the knowledge which has been accumulating regarding these 

 structures, and he notices that the first step towards complexity 

 is in the genus Euglena and its allies, in which the endoplast 

 becomes ovate in outline. A sausage shape is assumed in 

 some Ciliata, and its ribbon shape in Vorticella has long been 



known. In some of the Tentaculifera the nucleus is branched, and in some Ciliata, such 

 as Condylostoma patens (Fig. 12), it presents a necklace appearance, and in others the swellings 

 are widely separated by narrow processes. More than one endoplast exists in the Oxytrichidae, 

 one being in front and the other behind the centre of the body ; and in some species of Opalina 

 the endoplasts are numerous. In its more complex forms the endoplast is enclosed within a 

 very delicate transparent membrane. The nucleolus or endoplastule is sunken within the substance 

 of the endoplast in some forms ; it is attached to the inside of the membrane of the endoplast 

 in others, and on the outside in a few Infusoria. 



Two or three endoplastules exist in some, and in Vorticella they are granular fragments, one or 

 more of which become enclosed within each of the segmental portions into which the endoplast 

 becomes separated, during the process of internal budding, which will be noticed farther on. 

 The endoplast is in contact with the softer internal substance of the Infusoria (the endoplasm), and 

 also with the inner part of the cortical structures or ectoplasm. 



The Infusoria are usually more or less coloured, and the Flagellata, with rare exceptions, have 

 a small brilliant crimson spot at one end of the body ; in one genus there are two of the spots. 

 Amongst the Ciliata the red spot is rarely seen, and one genus has a black one ; but the Tentaculifera 

 do not have these pigment spots. Formerly they were considered to have to do with vision, but this 

 is an error, and the common term " eye spot," is therefore incorrect. Diffused colouring matter tints 

 284 



Fig. 12. COXDYLOSTOMA PATENS, SHOW- 

 ING MONILIFORM ENDOPLAST. 



