66 



ZOOLOGY FOR MEDICAL STUDENTS 



canal should it happen to have been properly situated in relation to 

 this lining at the moment extrusion occurred. 



A. Myxosporidia. This, the first sub-section of the Cnidosporidia, 

 includes a number of common parasites of fish — usually harmless 

 but occasionally causing destructive epidemics. They are to be found 

 creeping about amongst the tissues of the body or within its cavities 

 such as the urinary bladder or the gall bladder. They are more or less 

 Amoeba-like organisms which creep by lobopods but which do not feed 

 by them — nourishment being absorbed in solution by the general external 

 surface. There is a distinct clear transparent ectoplasm and a granular 

 endoplasm containing numerous nuclei. Within the endoplasm are 

 produced the spores — by complicated processes, accompanied by sexual 



fusion of nuclei, which need not be described 

 in detail. Normally the process of spore- 

 formation goes on continuously but it may 

 be concentrated in particular seasons. Thus 

 in Myxidium — a bright orange -coloured 

 parasite found creeping about on the lining 

 of the urinary bladder of the Pike — the 

 process is almost confined to the summer 

 months while in winter the creature re- 

 produces actively by separating off bud-like 

 outgrowths from its surface. 



B. Microsporidia. This group includes 

 a number of very small intracellular para- 

 sites occurring occasionally in Fishes but 

 far more usually in Arthropods. Some of 

 them are of practical interest as causing destructive epidemics in animals 

 of economic importance (Nosema bombycis — Silkworm disease, N. apis 

 — Bee disease). They are distinguished from Myxosporidia by their 

 possessing only one polar capsule and filament. 



(5) Sarcosporidia. This group includes a number of intracellular 

 parasites of the higher vertebrates, especially mammals, which are ex- 

 tremely common but the life-history of which is still very imperfectly 

 known. The young parasite in the form of an amoebula makes 

 its .way into the interior of a muscle-fibre and absorbing nourish- 

 ment grows actively, becoming eventually free from the muscle- 

 fibre and taking the form of a long tendinous-looking thread or tube, 

 reaching a length of it may be 16 mm. (Sheep) or even 50 mm. 

 (Roe-deer). The parasite becomes enclosed in a distinct envelope 

 which extends into its interior dividing it up into numerous chambers. 



Fig. 27. 



Cnidosporidia. A, Spore with 

 its two polar capsules ; B, a 

 separate polar capsule ; C, a polar 

 capsule with its tube extruded. 



