152 THE SPOROZOA 



for ingesting or digesting solid food, so frequent in other Protozoa, 

 are ever found in this group. Many Sporozoa possess flagella 

 during certain phases of the life -cycle, and many exhibit the 

 power of executing amoeboid movement and emitting pseudopodia 

 during even the whole period of growth ; but in both cases the 

 flagella or pseudopodia are organs of locomotion and not of 

 nutrition, except perhaps in so far as the latter may contribute 

 to an increase of the absorptive surface of the body. More 

 usually all such locomotor organs are absent, and the body of 

 the parasite has a fixed form and definite contours, limited ex- 

 ternally by a cuticle of greater or less thickness, through which 

 food is absorbed by diffusion. Food - vacuoles or contractile 

 vacuoles are never found. 



In the second place, the Sporozoa always possess the power of 

 rapid multiplication by sporulation that is to say, by the formation 

 of reproductive bodies or germs, each a fragment of the parent 

 body, in the form of a nucleated protoplasmic corpuscle, usually 

 very minute. These germs may serve for increasing the numbers 

 of the parasite within the same host, or may be the means of dis- 

 seminating the species and infecting other hosts. In the latter 

 case the germs are usually provided with protective envelopes 

 which enable them to leave the body of the host in which they were 

 produced and to endure for a season the vicissitudes of the outer 

 world. In some cases the protoplasmic germs are naked gymno- 

 spores, and all those derived from one parent are then enclosed in 

 a resistent cyst, formed by the parent previous to sporulation. 

 But in most cases the germs have their own special protective 

 envelopes, and are then termed chlamydospores, or more usually 

 spores simply. Within the spore-envelope a further multiplication 

 of the germs may take place, and a cyst enclosing all the spores de- 

 rived from a common parent may or may not be formed. Eesistent 

 spores of this kind are one of the most characteristic features 

 of this class, as the name Sporozoa implies. Only in the com- 

 paratively small number of cases in which infection is conveyed 

 from one host to another by an intermediate host, are protective 

 envelopes wanting. 



The bulk of our knowledge of the Sporozoa is of extremely recent 

 date, and great advances have been made during the last ten years in 

 the investigation of these organisms and the elucidation of obscure points 

 in their life -history. Nevertheless, they did not entirely escape the 

 observation of the earlier naturalists, even so far back as the eighteenth 

 century. As might have been expected, attention was directed first to 

 the larger forms of Gregarines inhabiting Arthropods, especially insects, 

 and later to the characteristic spores, often to be found in vast numbers 

 in various animals. 



The first notice of a Gregarine parasite is attributed to the famous 



