PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 127 



ment of later developmental stages is sharply marked off by a much lower 

 susceptibility from the segment-forming region immediately anterior to 

 it, except in individuals in which a new posterior end has recently de- 

 veloped in connection with fission (Fig. 42, C). In this respect, also, dif- 

 ferential susceptibility parallels differential dye reduction. Inversion of 

 susceptibility gradients has been observed in Dero and some other micro- 

 drilous species kept in clear standing water in diffuse light for a week or 

 two. Susceptibility to cyanide is decreased but increases from the two 

 ends toward a region anterior to the middle. Evidently the animals are 

 differentially susceptible to the unfavorable environment, and the differ- 

 ential effect on physiological condition inverts differential susceptibility to 

 cyanide. 



The electric-potential gradient of earthworms tested also shows two 

 opposed arms, at least on the ventral side, both ends being galvanometri- 

 cally negative to an intermediate region (Morgan and Dimon, 1904; 

 Watanabe, 1928). The galvanotactic reaction of the earthworm and of 

 several species of polychetes is also significant in this connection. When 

 exposed to the current, the animals bend into a U-shape with the two ends 

 toward the cathode. ^^ If the current is strong or if the animals are exposed 

 for a long time, a rather sudden reversal has been observed in a number of 

 species, both ends turning toward the anode. If these animals are removed 

 from the current for 15-30 minutes, they recover, and both ends are again 

 cathodic on renewed exposure. 



Water content, distribution of setae, and pigmentation show a U-shaped 

 distribution in various species of earthworms, according to several au- 

 thors; but an increase in water content from anterior to posterior levels 

 has been reported in two species." The difference in temperature at which 

 heat-shortening occurs at different body-levels has been determined for 

 several species of annelids. The differences are characteristic for the spe- 

 cies, showing in some a simple, in others a U-shaped, in still others an in- 

 verted U-shaped, gradient and in some a more varied differential. In the 

 earthworms tested, these temperature gradients are U-shaped and corre- 

 spond closely to the other gradient expressions. Their relations to other 

 conditions at different body-levels in polychetes are not known ; they may 

 perhaps correspond to regional differentiations (Hatai, 1924a; Watanabe, 

 1928). 



Few observations have been made on susceptibility gradients in adult 



" Moore and Kellogg, 1918; Hyman and Bellamy, 1922; Moore, 1923. 



"Hatai, 1924a; Kopenhaver, 1937, water content. Hatai, 19246; Sivickis, 1930, setae. 

 Pickford, 1930, pigmentation. 



