244 THE PROTOZOA 



filamentous pseudopodia, by means of whioh it is able to digest food in the 

 ordinary holozoic manner. The body is a plasmodium containing, in addition 

 to numerous nuclei, chromatophores, and peculiar "oat-shaped bodies," 

 "spindles," or "physodes," stated to consist of phloroglucin. The cyst- 

 envelope consists of cellulose, and has a stratified structure. In addition to 

 reproduction by fission (plasmotomy), Chlamydomyxa appears to form flagel- 

 late swarm -spores, possibly gametes. 



Labyrinthula occurs in marine and fresh water. In the active state it has 

 the form of a network of filaments, 1 millimetre or so in extent, over which 

 travel a great number of " units," each a nucleate cell or amcebula, sometimes 

 brightly coloured. When dried, each unit encysts and hatches out again 

 separately. The units multiply by fission. They were formally compared 

 erroneously with the ; ' spindles ' of Chlamydomyxa. Lister (298) regards 

 Ldbyrinihula as a colonial organism of which the units remain in connection 

 by their pseudopodia. He considers these two genera as related in one 

 direction to certain members of the Foraminif era (Gromiidce), in other drections 

 to the Heliozoa and the Proteomyxa. 



V. HELIOZOA. 



The Heliozoa are characterized, as a group, by their spherical 

 form and stiff, radiating pseudopodia, whence their popular name 

 of " sun-animalcules." As in the case of the Radiolaria, these 

 peculiarities of form are generally correlated with a floating habit 

 of life, though in a few cases the animal is sedentary and attached 

 to a firm support. In contrast with the Radiolaria, a " central 

 capsule' 1 (p. 250) is absent from the body-structure. A skeleton 

 may be present or absent. The majority of species inhabit fresh 

 water, but a few are marine. 



General Characters. As in other orders of Sarcodina, a concise 

 statement of the characteristic features of the group is rendered 

 difficult by the occurrence of border-line forms, of which the exact 

 position is doubtful. It is best, therefore, to consider first typical 

 forms of which the position is incontrovertible, and then those 

 which link the Heliozoa to other groups of Protozoa. 



The body-protoplasm exhibits commonly a vacuolated, frothy 

 structure, with distinct cortical and medullary regions. The cor- 

 tical zone, distinguished by vacuoles of larger size, disposed in a 

 radiating manner, is regarded as ectoplasm ; the medulla/ry region, 

 with smaller vacuoles irregular in 1 arrangement, as endoplasm ; 

 but it is open to doubt if these two regions correspond truly to the 

 ectoplasm and endoplasm of an amoeba. The cortex contains the 

 contractile vacuoles, and gives off the pseudopodia, which are 

 typically stiff, straight, and filamentous, ending in a sharp point 

 and supported by an axial organic rod (p. 48) ; but in some genera 

 the supporting axis is wanting. In the medulla are lodged the 

 nuclear apparatus, the food- vacuoles, and frequently also symbiotic 

 organisms, which are probably in most cases vegetative, non- 

 flagellate phases of holophytic flagellates (Chlamydo monads). 



As regards the nuclear apparatus, there are two types of arrange- 



