HEL10ZOA 73; 



from the usual method. A transverse constriction divides the body 

 into two halves as shown in two individuals of colony A each half 

 possessing its own nucleus and contractile vesicle ; the posterior seg- 

 ment, which at first lies free at the bottom of the cell, then pi-esses 

 forwards towards its orifice, as shown at C, and finally, by amoeboid 

 movements, escapes from it, sometimes stretching itself out like a 

 worm (as seen at D), sometimes contracting itself into a globe, and 

 sometimes spreading itself out irregularly over the pseudopodia of 

 the colony. But it finally gather.^ itself together and takes an oval 

 form ; and either develops a pair of iiagella, and forsakes the colony 

 as a free-swimming monad, or assumes the form of an Actinophrys, 

 moving about by three or four pointed pseudopodia probably in 

 each case coming after a time to rest, excreting a shell, and laying 

 the foundation of a new colony. There is reason to think that a 

 multiplication by longitudinal fission also takes place, in which the 

 escaping segment and the one left behind in. the old shell remain 

 attached by their pseudopodia, and the former develops a new shell 

 without undergoing any change of condition. 



Heliozoa. 1 The Actinophrya sol, sometimes termed the - sun- 

 animalcule ' (fig. 573), is one of the commonest examples of this group, 

 being often met with in lake>, ponds, and streams, amongst Conferva 3 

 and other aquatic plants, as a whitish -grey spherical particle dis- 

 tinguishable by the naked eye, from which (when it is brought under 

 sufficient magnifying power) a number of very pellucid, slender, 

 pointed rods are seen to radiate. The central portion of the body is 

 composed of homogeneous sarcode, inclosing a distinct nucleus ; but 

 the peripheral part has a 'vesicular' aspect, as in the type next 

 to be described (fig. 574). This appearance is due to the number 

 of l vacuoles ' filled with a watery fluid, which are included in 

 the sarcodic substance, and which may be artificially made either 

 to coalesce into larger ones or to subdivide into smaller. A ' con- 

 tractile vesicle.' pulsating rhythmically with considerable regu- 

 larity, is always to be distinguished, either in the midst of the 

 sarcode body, or (more commonly) at or near its surface ; and 

 it sometimes projects considerably from this, in the form of a 

 sacculus with a delicate membranous wall, as shown at fig. 573, 

 A, cv. The cavity of this sacculus is not closed externally, but 

 communicates with the surrounding medium not, however, by any 

 distinct and permanent orifice, the membraniform wall giving way 

 when the vesicle contracts, and then closing over again. This alter- 

 nating action seems to serve a respiratory purpose, the water thus 

 taken in and expelled being distributed through a system of channels 

 and vacuoles excavated in the substance of the body, some of the 

 vacuoles which are nearest the surface being observed to undergo 

 distension when the vesicle contracts, and to empty themselves 

 gradually as it refills. The body of this animal is nearly motionless, 2 



1 A systematic account of this group is to be found in Dr. F. Schaudiim's 

 'Heliozoa,' the first part of the comprehensive Das T/iterreicJi, edited by the 

 German Zoological Society, Berlin, 189(5. M. Peiiard's memoir, ' Etudes sur quelques 

 Heliozoaires d'Eau Douce,' in vol. ix. of the Arch, de Biol., should be consulted. 



2 A swimming Heliozoiin has lately been described by M. E. Pennrd, who calls it 

 Myrioplinjs paradn.ru . 



3 B 



