NONPHOTIC RECEPTORS IN LOWER FORMS 



373 



FIG. 7. Section of the last two segments of the antennae 

 (7 and 8) of the diplopod Polydesmus complnnatus. ep, epidermis; 

 fg, finger-like organ (function unknown); h, sensory hair; 

 j, skin joint; mu, muscle; n, nerve; p, peg-Hke sensilla; sci, sc;, 

 and SC3, the three types of sense cells; tr, trichogen cell. [From 

 Plate C92).] 



are found in great numbers in all soft skinned inverte- 

 brates. Up to the present they have not been found in 

 the turbellarians and echinoderms — probably for 

 technical reasons. As a rule, these peripheral fibers 

 form a plexus which can be located subepithelially in 

 the connecti\e tissue or subcuticularly above the epi- 

 thelium. Such plexuses were first described in the 

 classical works by Retzius (106) and von Apathy 

 (129). In arthropods such neurons with free terminals 

 are limited to the soft skin of the joints and that be- 

 tween the segments (fig. 8). However, they are found 

 also in the epithelium (hypodermis) ol the mouth 

 parts (fig. 9). The cells 01 these neurons are mostly 

 located at some distance from the terminal ends of 

 the dendrites. Sometimes, as in the above mentioned 

 stretch receptors of the crustaceans, these dendrites 

 are short. 



The simple type of receptors is common in inverte- 

 brates. On the other hand, some very complicated 

 sense organs are found, for example the phonore- 

 ceptors and eyes of the insects, the eyes of the octopus, 

 etc. Even in its highest form, however, the complexity 

 never reaches that of the vertebrates. 



ch.o 



different chemical substances; one neuron reacts only 



to sugar (with spikes of smaller voltage), the other one 



(with greater spikes) to salts, acids and alcohols (60). 



Neurons with free nerve endings in the epithelium 



FIG. 8. The trochanter joint of the third pair of extremities 

 oi an Aeschna larva, bp, bipolar sensory neuron; ch.o., chordo- 

 tonal organ with bipolar sensory neurons; co, coxa; fe, femur; 

 sc, sensory neuron with dendritic terminals at the joint; sh, 

 sensory hairs; tl, trochantinus. [Redrawn after Zawarzin (144).] 



