336 THE BIOLOGY OF MARINE ANIMALS 



marine invertebrates. In the anemone Calliactis parasitica, the oral disc 

 is at least 4,000 times as sensitive as the column to mechanical stimulation; 

 also sensitivity decreases from the base of the column upwards. Now, 

 tactile receptors of Calliactis lie in the column endoderm, the mesogloea 

 of which increases in thickness from pedal edge towards the marginal 

 sphincter. The decrease of sensitivity in the same direction corresponds 

 to this increase of mesogloeal tissue which exerts a shielding effect (119). 

 Tubicolous polychaetes are most sensitive to contact in the region of the 

 gills. In Hydroides dianthus (Serpulidae) the order of decreasing sensitivity 

 for various regions of the body is as follows: gills, head, thorax and ab- 

 domen. In Holothuria decreasing sensitivity is shown by: tentacles, oral 

 rim, cloacal rim, podia, anterior body region, posterior end and mid-body 

 surface. 



Contact stimuli can often be localized with great precision. Foreign 

 bodies are deftly removed from the external surface ; urchins, when attacked, 

 direct their spines towards the region affected, etc. In quiescent spinal 

 dogfish and teleosts a localized tactile stimulus throws the body into an 

 S-shape, the posture of which depends on the position of stimulation. A 

 touch anteriorly causes the tail to move to the opposite side; a touch 

 posteriorly, to the same side. Active spinal preparations of the dogfish 

 show persistent locomotory rhythms as long as they are free from contact, 

 while diffuse contact stimulation of the ventral surface produces inhibition 

 of swimming movements. These various kinds of responses to stimulation 

 are instances of reflexes involving peripheral nerve nets or central nervous 

 systems {vide Chapter 10). The neurological basis of locomotory rhythms 

 and reflexes in selachians is considered in detail by Lissmann (96). 



By recording from the facial nerve of the catfish (Ameiurus) Hoagland 

 (73) has picked up action potentials following mechanical stimulation. 

 Receptors (presumably free nerve endings of Gasserian origin) in lips and 

 barbels are very sensitive to touch and water movements. The spikes are 

 large, indicative of large axons; quite distinct from these are small 

 spikes, produced by chemical agents and transmitted in small fibres of the 

 geniculate ganglion. 



Pressure receptors in the skin of selachians are of two sorts : free nerve 

 endings and terminal corpuscles. The latter are encapsulated skeins of 

 nerve fibres lying in the connective tissue of the fin (Fig. 8.1). When stimu- 

 lated by pressure they give rise to bursts of impulses in the sensory nerves. 

 Adaptation is slow, and discharge continues for many seconds under 

 maintained steady stimulation. The terminal corpuscles respond to fin 

 movements as well as externally applied pressure, and thus probably act 

 as proprioceptors as well as tactile receptors (99). 



PROPRIOCEPTORS 



Proprioceptors are mechanoreceptors that respond to stretch, bending and 

 contraction, and provide information about the movement of body parts. 

 Those best known are the muscle spindles of higher vertebrates, but stretch 



