EFFECTOR MECHANISMS 



373 



filament which in some species reaches a length fifty times that of the spore 

 (Fig. 9.2). Exposure to the digestive fluids of the host causes the contained 

 thread to unroll and shoot out. The discharged threads serve temporarily to 

 anchor the parasite to the gut wall of the host (27, 30, 49, 56, 59, 101). 



Nematocysts and Colloblasts 



Nematocysts or stinging cells are found in coelenterates. Colloblasts 

 or adhesive cells occur in ctenophores. 



The nematocysts of coelenterates are small capsules lying in cells known 

 as nematocytes or cnidoblasts. The capsules are minute structures, usually 



Fig. 9.3. Nematocysts (Holotrichous Isorhizas) of Corynactis 



a, capsule and base of shaft after discharge (fresh smear in distilled water), b, nemato- 

 cyst before discharge (dried smear in distilled water), c, shaft of an incompletely dis- 

 charged nematocyst, showing the tip; a-d, focused in different planes (dried smear in 

 distilled water). (From Robson (107).) 



some 5-50 jli in length, and fusiform to spherical in shape. Within the 

 capsule lies coiled a hollow thread which can be everted (Fig. 9.3). An 

 operculum caps the nematocyst and there is often a bristle-like cnidocil 

 on the outer surface of the cell. Other structures frequently present are 

 supporting rods about the periphery of the cnidoblast, and a fibrillar 

 network associated with the capsule. 



There are two main types of nematocysts ; the spirocysts of Zoantharia, 

 and nematocysts proper found in all groups of coelenterates. The spiro- 

 cysts have thin-walled capsules permeable to water, and they discharge 

 unarmed adhesive threads. The nematocysts proper comprise a large 

 variety of forms, distinguishable by characters of their discharged threads. 

 All of them have thick-walled capsules impermeable to water except at 

 discharge, and containing threads that are usually armed with spines 



