NONPHOTIC RECEPTORS IN LOWER FORMS 



371 



a well developed cuticle. The distal end of the thread 

 of the stenoteles is open and by this means the poison 

 stored in the capsule can be injected through the 

 thread. The desmonemes on the other hand have a 

 thread which is closed at its distal end and it winds 

 only around the spines and other parts of the food 

 organisms. 



2) The atrichous isorhizas (also called small glu- 

 tinants)^erve Hydra by attaching the tentacles to the 

 ground during the migration of the polyps. 



5) Finally the holotrichous isorhizas are exclusively 

 a defense mechanism. They e.xplode only upon types 

 of stimulation which cause no feeding reaction. 



The discharge of the nematocysts occurs only upon 

 direct stimulation; no nervous control exists. No nerve 

 fibers can be found which lead to the cells containing 

 nematocysts. With electrical stimulation only the 

 nematocysts directly stimulated react (88, 90). Even 

 repeated rhythmic stimulation by means of condensor 

 discharges never causes a diflfusion of excitation be- 

 yond the area direc-tly stimulated. Thus the cnido- 

 blasts contain irritable structures which act both as 

 receptors and effectors and are independent of a 

 nervous system. 



Direct mechanical stimulation of the nematocysts 

 on the tentacles of Anemonia or of the penetrants and 

 volvents of Hydra does not normally lead to a dis- 

 charge even though the cnidocils present are diverted 

 (for example by epibiotic protozoa or artificially by 

 chemically very clean, rounded glass needles). Neither 

 is discharge obtained by chemical stimulation alone 

 (such as extracts of meat or food organisms, proteins, 

 amino acids or sugar). However the threshold for 

 direct mechanical stimulation is considerably reduced 

 by chemical stimuli produced by the food. The im- 

 mediate releasing stimulus is therefore a mechanical 

 one which however only becomes efTective if the 

 threshold is lowered in advance by certain substances 

 present in the food. 



The nature of these very specific chemical sub- 

 stances present in the food organisms is unknown. 

 They are not proteins but they are firmly adsorbed on 

 the proteins; they can, however, be extracted with 

 ethanol or acetone (88). 



The atrichous isorhizas serve to attach the tentacles 

 to the ground during the inigration of the polyps. 

 They never respond to stimuli arising from food or- 

 ganisms. Chemical stimuli such as extracts of food 

 organisms raise the threshold for this type of nemato- 

 cysts. The duration of mechanical stimulation neces- 

 sary to bring about discharge is greater for atrichous 



isorhizas than for stenoteles. Food inhibits chemically 

 the discharge of the atrichous isorhizas (36). 



In summary, it may be concluded that nematocysts 

 respond to a mechanical stimulus. A simultaneous 

 chemical stimulus, by raising or lowering the thresh- 

 old, determines which kind of mechanical stimulus 

 will explode the nematocysts. The change of the 

 threshold insures that the reaction will be appropriate. 



The cnidoblast is therefore a unique tissue element. 

 As an independent effector it contains sensory, excitor 

 and effector elements. The sensory element is in itself 

 not simple and functions by means of two distinct 

 sense organs, mechanical and chemical in nature. The 

 cnidae may in fact be said to be double sense organs 

 as well as effectors. There are no obvious analogies 

 to this in the tissues of higher animals (88). 



HIGHER INVERTEBR.-kTES: EMERGENCE 

 OF TRUE RECEPTORS 



Anatomical Peculiarities 



The receptor cells of the invertebrates are always 

 primary sense cells; every sense cell has therefore a 

 centripetal afferent nerve fiber. This is also the case 

 in organs which in vertebrates have secondary re- 

 ceptor cells, for example the static and auditory 

 organs. 



In the simplest case, the sense cells are separate and 

 are not yet united into an organ. Such scattered sense 

 cells are found in all classes of invertebrates. They 

 have the simplest shape in hydroid-polyps and actin- 

 iae; here they are located in the epithelium and have 

 the shape of epithelial cells. They appear in the ecto- 

 derm as well as in the entoderm (fig. 3). They may 

 be absent in the column ectoderm of the actinians, 

 even if they are numerous in the ectoderm of the oral 

 disc (88). Nevertheless, the column is sensitive to 

 mechanical stimuli from the en\ironment, although 

 4000 times less so than is the oral disc (91). Such 

 single sense cells are found in the epithelium of lower 

 and higher wonns and molluscs, e.g. Lumbricus as 

 shown in figure 4. The sense cells of the higher in- 

 vertebrates are normally located subepithelially and 

 send one peripheral fiber into the epithelium. These 

 bipolar sensory neurons are illustrated in figure 5. 



These single sense cells may ha\'e an auxiliary ap- 

 paratus; for example, the hair-sensillae of the arthro- 

 pods. These often, but not always, contain only one 

 sense cell which sends a peripheral fiber into the in- 

 terior of a hair which was formed bv the cuticle 



(fig- 13)- 



