464 STANLEY C. BALL 



in late autumn as in summer at Le Portel, nor does he find 

 them less' numerous in December and April. The greatest 

 number of P. gemellipara obtained at New Haven was collected 

 on November 9, 1912, when about 175 mussels yielded 310 

 worms. Only those easily detected with the naked eye were 

 taken. 



Linton states that he found this species negatively photo- 

 tropic; that on leaving a dish containing specimens in strong 

 light the worms were afterward found to have collected on the 

 side opposite the source of light. My observations, on the other 

 hand, point to a neutral reaction on the part of the older worms 

 and a positive reaction of the younger. The adults are as often 

 found on the light side of the dish as on the dark. Their ob- 

 served tendency to secrete themselves beneath debris on the 

 bottom of the dish was at first interpreted to signify that they 

 were reacting negatively to light, but when it was ascertained 

 that in a dish lacking such debris these older individuals seemed 

 neutral it was concluded that thigmotropism was responsible 

 for their hiding reactions. It has been noticed that an adult 

 worm, upon coming in contact with a piece of mussel gill or 

 other tissue, often crawls along in contact with it. 



The young worms, on the other hand, are positively photo- 

 tropic. If a number of them be placed in a watch glass and the 

 latter then rapidly rotated so as to cause the collection of the 

 worms at the center, as the water comes to rest they immediately 

 swim vigorously toward the source of light. Only daylight 

 was used. This experiment was made more striking by the in- 

 troduction into the watch glass of several specimens of Aphano- 

 stoma. The latter, being negatively phototropic, as rapidly 

 crawled to the darker side of the glass. Repeated trials were 

 followed by the same reactions, the two groups of worms sepa- 

 rating at the center and progressing in opposite directions. 



There is a marked tendency on the part of the young worms 

 to crawl up the side of the dish above the water line; death 

 often resulted. 



One can conceive that thigmotropism may be of value in 

 causing the older worms to remain in the mussels. Linton 



