MOTOR RESPONSES IN INVERTEBRATES 587 



on opposite sides, but nothing is known concerning the details (Mast, 



140). 



The planulae of Eudendrium, small ciliated organisms which move 

 slowly on the substratum, also turn directly toward the light. The 

 anterior end continuously swings from side to side, but there are no 

 shock-reactions. No details concerning the process of response are known 

 (Mast, 140). 



WORMS 



Some of the worms are very sensitive to light and respond very 

 precisely, especially in the larval stages, but only a few species have been 

 thoroughly investigated. I shall present the results obtained with these 

 species in some detail and those obtained with some other species very 

 briefly. 



TUBICOLOUS ANNELIDS 



Many of the worms give very striking shock-reactions, usually to rapid 

 decrease in luminous intensity. Various tubicolous annelids are par- 

 ticularly noteworthy in this respect. If a shadow is cast on them, they 

 suddenly dart into their tubes (Dalyell, 46; Ryder, 191; Andrews, 7; 

 et al). This response doubtless serves to protect them against enemies. 

 It is readily modified, and the amount of reduction in light necessary to 

 induce it varies greatly, as does also the time the animals remain in the 

 tubes after a response. Mrs. Yerkes (233) found, in a series of 60 suc- 

 cessive responses of a specimen, that it remained in the tube from 10 to 

 710 sec. It is consequently evident that these responses are not closely 

 correlated with the magnitude of the change in the light received. They 

 are due to muscular contractions, but nothing is known concerning the 

 photoreceptors, the internal processes involved in the contraction, and 

 the stimulus and the impulse that induce it. 



ARENICOLA LARVAE 



The larvae of many aquatic worms are at first intensely photo- 

 positive and later photonegative. Observations made on the larvae of 

 Arenicola lead to the following conclusions: These larvae are finger-like 

 in form and about 0.3 mm. long. They have two eyes and a band of 

 cilia near either end. They swim for a time after they hatch, then 

 settle to the bottom and crawl. They orient precisely and are strongly 

 positive when they swim and negative when they crawl. 



Swimming is the result of ciliary action, but orientation is the result 

 of muscular contraction. The larvae rotate on the longitudinal axis when 

 they swim, so that when they are laterally illuminated, the two eyes are 

 alternately directed toward and away from the light, resulting in 

 alternate increase and decrease in the intensity on them. When in the 



