//. TYPES IN DETAIL IVITH THEIR SUBORDINATE GROUPS. 7 



this place. In many cases it is impossible to tell with certainty whether 

 the organism in question is a plant or an animal, and the decision rests 

 upon a common agreement rather than upon any definite structure or 

 physiological reaction. 



Class IV. SPOROZOA Parasitic Protozoa with a compli- 

 cated life-history, one stage of 

 which is usually a cyst or case con- 

 taining numerous minute individ- 

 uals called spores. Recently this 

 class has acquired a great though 

 unpleasant notoriety through the 

 establishment of the fact that sev- 

 eral of our worst diseases, such as 

 malarial and yellow fevers, are 

 caused by members of this group, 

 one stage of which is passed in 

 the human blood, while various 

 species of mosquito serve as the 

 intermediate host. 



X Monocystis (in earthworms). 

 Hcsmamoeba (in blood of verte- 

 brates and in insects). 



TYPE II. CCELENTERATA. 



Animals built on the plan of a Gastrula, i.e., that of a cup or 

 vase with double walls, ectoderm and endoderm ; with a large cen- 

 tral cavity, the gastrocoele, and with a mouth or protostome. They 

 are strictly radial in structure and possess a central axis and two 

 poles, oral and apical. The parts are arranged about the axis in 

 2\ 4', 6' and their multiples. A fundamental difference in the 

 three sub-types, although seen in some of them only in the embryo, 

 is the difference in development and use of the apical pole. 



In the sessile Cnidaria it is used as the point of support, and is 

 non-sensitive in the free-swimming ones; in the Clenophora it is di- 

 rected forward during locomotion and bears the principal sense- 

 organs; and in the Porifera it is the point at which the excurrent 

 orifice, the osculum, is situated. 



