370 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



taken necessarily as proof that the posterior part of 

 Spirostomum is not irritable to chemicals. This form 

 responds to other chemical and mechanical stimuli in 

 the anterior and posterior part. 



A localized, receptorlike structure has so far been 

 found in only a few flagellates. Many phytoflagellates 

 QEuglena and others) possess an eyespot which con- 

 tains the pigment astaxanthin (135). In Euglena there 

 is a light sensitive plasma spot located in the con- 

 cavity of this eyespot. The eyespot itself probably has 

 a screening function to light (81, 84). There are other 

 phytoflagellates (e.g. Chlamydomonai) which are sensi- 

 tive to light throughout the body, even though they 

 possess an eyespot, as shown by mutants without an 

 eyespot (52). Therefore the question as to whether 

 the eyespots are real receptors is still open. 



The effector system of the protozoa is very compli- 

 cated. It is certain that in no case do protozoan cells 

 react according to the all-or-none principle. The de- 

 gree of contraction of the pseudopodia of rhizopods 

 depends on the intensity of stimulation; the stronger 

 the stimulation the farther the contraction spreads 

 along the excited p.seudopodium; it will spread to 

 other parts of the body if the stimulus is sufficiently 

 strong (127). The contractile stalk of vorticellae can 

 be completely or partially contracted depending on 

 the intensity of the stimulation (29, 64). The rhythm 

 of ciliary movements in some parts of the body of 

 ciliates can be modified independenth of the activity 

 of the remaining cilia (65, 89). 



COELENTER.JiTES: GNIDQBL.ASTS .■XS 

 INDEPENDENT EFFECTORS 



The cnidoblasts are cells which are characteristic of 

 the coelenterates (Cnidaria, polyps and medusae). The 

 intracellular structures of the cnidoblasts are nemato- 

 cysts which consist of a bubble-like capsule. The free 

 pole is a long hollow thread which is introverted and 

 coiled, as shown in figures i and 2. The opening of 

 the capsule is usually covered by a cap. On discharge 

 the cap bursts open and the thread is ejected by 

 eversion. Different types of nematocysts are found in 

 the same species. Furthermore, they are different in 

 different species (56, 63, 118, 138). 



So far as analysis with ordinary light microscopes 

 is concerned, nematocysts are the most complicated 

 structures formed by cells. Some cnidoblasts carry a 

 fine spine or a cone of fused cilia on the free end, the 

 cnidocil shown in figure i ; others lack this cnidocil. 

 Cnidoblasts which are not yet differentiated (inter-- 



stitial cells) form new nematocysts during their whole 

 life; the cnidoblasts migrate — sometimes in groups — 

 into or between ectodermal cells and thus form bat- 

 teries of nematocysts. 



There exists a large number of morphologically 

 different nematocysts (138), but only a few types 

 have been analyzed physiologically. They show char- 

 acteristic differences with respect to irritability and 

 function. 



/) The desmonemes (also called volvents) and 

 stenoteles (also called penetrants) are used for catch- 

 ing food. They explode upon simultaneous stimulation 

 by chemical and mechanical means (such as aquatic 

 food organisms and meat); the cap bursts open and 

 the thread is everted within 3 to 5 msec. The thread 

 of the stenoteles, supported by spines at the base, 

 penetrates the body of the food organism even through 



FIG. I. Scheme of a stenotele nematocyst and its discharge, 

 a, cnidoblast (ez) with nucleus (n), nematocyst (st), and 

 cnidocil (en); b, stenotele nematocyst during discharge; c, after 

 discharge showing cap (i), spine (sp), and ejected thread (th). 

 The cnidoblast is not drawn in b and c. Magnification, 555 X. 

 [From Kiihn & Schulze (76).] 



FIG. 2. Nematocysts of Hydra (cnidoblasts omitted), a, des- 

 moneme prior to discharge; b, same after discharge; c and d, 

 atrichous isorhizas. Magnification. 2200 X. [From Kiihn & 

 Schulze (76).] 



