ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 191 



major pathways of nervous transmission are, by contrast, unusually 

 clear and well defined, {a) Tactile receptors are absent from the shell 

 surfaces. The " scales " and " hairs " upon the girdle are important 

 tactile organs. The ctenidia are also sensitive to touch, as are the 

 proboscis, the foot, and the ventral surface of the girdle. The foot is 

 positively thigmotactic to large surfaces, but retracts locally when 

 stimulated by a small surface. The tegmental aesthetes are photo- 

 sensitive ; they are activated by light of constant intensity and by 

 sudden decrease in light intensity, not by increase. The dorsal surface 

 of the girdle, the soft ventral surfaces and the periphery of the girdle 

 are sensitive to light. The superficial soft tissues are open to 

 chemical activation, to stimulation by abnormal osmotic pressures, and 

 by " irritants." Tactile, photic, and chemo-receptors are physiologically 

 distinct. There is no clear evidence of sensitivity to heat ; that to cold 

 is less doubtful. There is a pronounced tendency for the animal to 

 come to rest in positions avoiding uneven tensions in the musculature. 

 This is responsible for the precise negative geotropism exhibited by Chiton. 

 It is not sensitive to vibratory mechanical disturbances, (b) The 

 problem of differential irritability is difficult. One factor is anatomical 

 isolation of particular receptors (e.g. removal from the external surface). 

 Another is the structure of the receptor. An additional factor is 

 probably found in the possession by certain receptor cells of special 

 substances which enter into excitation reactions. On the sole of the 

 foot of Chiton there is evidence of separate photo-, tacto-, and chemo- 

 reception. Even if the epithelial cells of an animal were open to sensory 

 activation by a variety of stimuli, it would not be legitimate to argue to 

 a primitive " universal " kind of receptor, (c) The reactions of Chiton 

 to local stimulation are of a character consistent with the known 

 distribution of the central nervous system. The responses of isolated 

 portions of an animal cut transversely are such as to show the absence 

 of any strong centralization. This is in agreement with the known 

 occurrence of ganglion cells throughout the whole length of the nerve 

 strands, (d) The young Chiton is photo-negative, the old Chiton 

 photo-positive, to sunlight. There is a progressive age change, which is 

 connected with the erosive destruction of the photo-sensitive aesthetes. 

 The erosion of the shell is due to growth effects and to organisms 

 settling down on it. (e) The homochromic coloration of Chiton is 

 determined by the algal food and by organisms on the shell. A homo- 

 chromically coloured Isopod is a characteristic companion. Various 

 harmonious correlations follow automatically in the wake of the changing 

 phototropism of Chiton. The animal's habits determine the environment 

 in which it lives. J. A. T. 



Homing of Limpet. — Henri Pieron {C. R. Soc. Biol, 1919, 82, 

 1227-30). The limpet shows a topographical memory for its site. It 

 feels the surface of the rock with its cephalic tentacles, and to some 

 extent by its pallial tentacles. But there is also a kingesthetic memory. 

 There are two other factors which operate in the homing — namely, 

 gravity and illumination— but these are secondary compared with the 

 topographical data afforded by touch on rough surfaces. J. A. T. 



