250 



THE PROTOZOA 



structure of the body, and especially the skeleton, may depart more 

 or less widely from the radiate symmetry which is to be regarded, 

 probably, as primitive for the group. Hence three principal types 

 of symmetry can be distinguished in these organisms : (1) Homaxon 

 (Figs. 13, 105, 107), in which all axes passing through the centre 

 are morphologically equivalent, the symmetry of the sphere ; 

 {2) monaxon (Fig. 109), in which the body has a principal or vertical 

 axis round which it is radially symmetrical, the type of symmetry of 

 the cone ; (3) bilaterally symmetrical (Fig. 106), in which the body 



fi 



FIG. 105. Acanthometra dastica, Haeckel. sp., Radiating spines of the skeleton 

 (twenty in number, but only twelve are seen in the figure) ; ps., pseudo podia ; 

 c., calymma ; ex., central capsule ; N., N., nuclei ; x, yellow cells ; my., myo- 

 phrisks. After Butschli, Leuckart and Nitsche's " Zoologische Wandtafeln." 



can be divided along a principal plane into equivalent right and 

 left halves. With further modification the body may become 

 asymmetrical. Sedentary forms are not known in this group, the 

 species of which are exclusively marine, and occur on the open 

 surfaces of seas and oceans, reaching in many instances a re'atively 

 large size and a very high degree of structural differentiation. 



In the internal structure, the most salient feature is the division 

 of the body by means of a membranous structure, termed the 

 central capsule (Fig. 13, CK), into a central medullary region and a 

 peripheral cortical zone hence distinguished as the intracapsular 



