4 GENERAL HTSTOEY OF THE INFUSORIA. 



Among the variations in the ends of the fusifonn or elongated genera may 

 be noticed the notched or emarginate apices of Tetmemorus (II. 12); the 

 truncate extremities of Docidium (II. 9, 10), sometimes also, as in D. 

 Ehrenbergii, tnberculate; and the more or less acutely conical apices of 

 Closterimn, prolonged in some species, as in 0. attemiatum, by an abrupt 

 contraction of the frond into a conical process — in others, as in C. setaceum 

 and G. rostratum, by the gradual tapering of the whole frond — into long 

 rostrate or setaceous beaks. 



Colour. — This is due to the endochrome or internal substance, which is 

 usually of a herbaceous green colour, and often diffused pretty uniformly 

 throughout the fronds, sometimes however leaving intervals at which the 

 enclosing membrane (lorica, Ehr.) becomes visible. This lorica is itself 

 mostly coloiuiess ; yet in several species of Closterium and Penium it has a 

 reddish-brown tint (II. 5, 6, 15). The green colouiing matter of the interior 

 is identical with that of plants, i. e. it is chlorophyll or chromule, and con- 

 sequently undergoes a change of coloui' in autumn, becoming, like the leaves 

 of plants at' that season, a reddish-brown. When this change occiu's, it is 

 equally indicative of the termination of hfe. 



Consistence. — Envelopes. — The hmiting membrane of Desmidiaceae is 

 firm, though flexible ; it exhibits some elasticity and considerable resistance 

 to pressure, is not brittle, and not readily decomposable. Traces of siHca are 

 found in a few species, but not, says Mr. Ralfs, " in sufficient quantity to 

 interfere with their flexibility." It is lined by a softer flexible membrane ; 

 and besides this, the Desmidieae generally have an external mucous or gela- 

 tinous covering, mostly so transparent and homogeneous as to be overlooked. 

 To bring it into view, it is a common plan to add some colouring matter to 

 the water in which the organism is viewed ; but good manipulation with a 

 high power will frequently succeed without recoiu-se to this expedient to 

 demonstrate it. The particles of colour diffused about the frond, and indeed 

 any external bodies, such as small vegetable cells, are seen, not in contact 

 with the fronds, as they would often be if these were naked, but kept at a 

 distance corresponding with the width of the hyahne envelope (I. 15 ; II. 35). 

 In Didymoprium GrevUlii and Staurastrum tumidum the mucous sheath is 

 distinct and well defined ; " in others (to quote Mr. Ealfs) it is more atte- 

 nuated . . . . , and, in general, its quantity is merely sufficient to hold the 

 fronds together in a kind of fihny cloud which is dispersed by the shghtest 

 touch. AATien they are left exposed by the evaporation of the water, this 

 mucus becomes denser, and is apparently secreted in larger quantities to 

 protect them from the effects of drought." 



The hning or the primordial membrane of the firm lorica is thin, colourless, 

 and highly elastic, and alters its contour with the varying movements of the 

 endochrome which it immediately invests. It is in contact with the outer 

 case only at some points, mostly about the centre, and being elsewhere free, 

 an intei-val exists between the two envelopes. This elastic lining is acted on 

 by various chemical reagents ; for instance, it is contracted or connigated by 

 iodine and by acids. 



Openings in Lorica. — Openings have been represented by several writers 

 in the finn envelopes of Desmidieae, and more particularly in those of Clos- 

 terium. Ehrenberg, for instance, stated that apertures existed at the extre- 

 mities, through which soft, veiy short, and conical transparent papiUae 

 shghtly protruded to serve as locomotive organs. Both Mr. Varley and Mr. 

 Daliymple also described tenninal orifices, closed within, however, by a mem- 

 branous envelope ; but neither they nor any other observers have detected 

 the papilla-Hke locomotive organs Ehrenberg represented. '^ In no instance 



