26 ZOOLOGY FOR MEDICAL STUDENTS chap. 



composed in great part of coin-shaped discs or nummulites ranging up 

 to more than two inches in diameter, each disc being really a huge 

 foraminiferal shell with successive chambers arranged in a spiral, and 

 showing the usual differentiation into microspheric and megalospheric 

 individuals. 



There are two groups of organisms — the Mycetozoa and the Proteomyxa 

 - — ^which are not specifically mentioned in the scheme of classification on 

 p. 1 8 but which must be briefly referred to now. 



The Mycetozoa (Myxomycetes of the botanists) constitute a group 

 which shows such a mixture of animal and vegetable characters that it 

 cannot be said definitely to have taken up either the one line of evolution 

 or the other. Mycetozoa are frequently seen in damp woods forming 

 a beautiful lace-work, often of a bright orange colour, on the surface 

 of fallen and decaying trees. There is no sign of life obvious to the 

 naked eye, except that continued observation shows that the patch of 

 lace-work slowly changes its position. If the network is allowed to 

 spread on to a moist glass slide and examined microscopically it is seen 

 to consist of actively streaming cytoplasm, showing a difierentiation into 

 ectoplasm and endoplasm and containing numerous nuclei. Whereas 

 a portion of protoplasm containing a single nucleus is termed a cell, 

 such an undivided mass of protoplasm as this containing a number of 

 nuclei is termed a syncytium or plasmodium. It has become advisable 

 to use the former term rather than the latter to avoid confusion owing 

 to the word plasmodium being used also in another sense (see p. 55). 



The Mycetozoa are organisms which have given up aquatic existence 

 and become more or less terrestrial, and, correlated with this, they have 

 developed special arrangements for protection against desiccation. 

 When subjected to drought the protoplasm segments up into numerous 

 pieces each surrounded by a protective cyst — the whole forming a hard 

 brittle mass. When conditions again become favourable the cysts 

 soften and the protoplasmic masses creeping out fiow together again. 

 When the reproductive period comes oh the protoplasm clumps together 

 into compact masses usually supported by secreted stalks attached to 

 bark or other solid substance. These " sporangia " are of characteristic 

 shape and colour in the different genera and species. The protoplasm 

 in their interior gives rise to numerous small reproductive bodies or 

 spores each enclosed in a hard protective cyst. These spores are blown 

 about by the wind and under favourable moist conditions the protoplasm 

 in their interior issues forth as a minute cell which swims for a time by 

 the movements of a single flagellum but presently takes on an amoeboid 

 character, and undergoes fission a few times. The amoeboid cells so 



