lOO THE INVERTEBRATA 



Subclass NEOSPORIDIA 

 Sporozoa in which the adult of the vegetative stage is a syncytium ; 

 which usually forms spores continuously within itself; and the spore 

 cases are usually complex structures, which, except in the Actino- 

 myxidea, contain only one germ. 



Order CNIDOSPORIDI A 



Neosporidia whose spores possess pole capsules. 



The formation of the spores in this group is a complex process of 

 which the details and the relation to the typical life cycle of the 

 Protozoa have not yet been completely elucidated. The following 

 scheme provisionally co-ordinates the facts that have been estab- 

 lished concerning it. In the syncytium (Fig. 82), which is the 

 agamont and which often multiplies by plasmotomy, there arise, 

 perhaps by the coming together of nuclei, bodies known 2iS pansporo- 

 blasts^ each composed of a couple of envelope cells with one or more 

 cells known as sporoblasts. The nucleus of each sporoblast divides and 

 the sporoblast gives rise to a complex, multicellular spore, composed 

 of a case of two or three pieces, each with an underlying nucleus, one 

 to five nematocyst-like pole capsules ^ each with a nucleus, and one or 

 more germs. In most cases the germ is single and at first has two 

 nuclei, which later fuse. Here we may regard the sporoblast as a 

 gamont and the products of its division as homologues of gametes, of 

 which some become the accessory cells of the spore and two (those 

 which the germ at first possesses) the definitive gametes. In one 

 group, however (the Actinomyxidea), there are several germs (often 

 as a syncytium), and syngamy takes place not between nuclei in a 

 germ but at an earlier stage, between pairs of cells in the pansporo- 

 blast, each zygote becoming a sporoblast. Here the sporoblast is a true 

 sporont, and the products of its division are homologues of sporozoites, 

 of which some become the accessory cells of the spore and the others 

 (the germs) are the definitive sporozoites. It is a remarkable, but 

 apparently an established, fact, that syngamy thus takes place at 

 different stages in the formation of essentially similar spores. 



Infection of new hosts is by the mouth, and the function of the 

 pole capsules is, by discharging their threads, to anchor the spore to 

 the gut wall. A schizogony may precede pansporoblast formation. 



Of the three suborders of the Cnidosporidia, the Myxosporidia have 

 two or four pole capsules in the spore, the Microsporidia one, and the 

 Actinomyxidea three. The latter group also differ from the other two 

 in respect of the germs, as mentioned above. 



Myxobolus (Myxosporidia, Fig. 82). Large syncytia in the tissues 

 of various freshwater fishes. Some species are harmless, others 

 dangerous pests. 



