THE BIONOMICS OF CONVOLUTA ROSCOFFENSIS. 395 



the parts played by tliem in determining the habits of the 

 animal. 



1. Geotropism. 



The young Convoluta, the moment after hatching, reacts 

 to gravity like the adult. In a perfectly still vessel Con- 

 voluta is negatively geotropic : it rises to the surface of 

 the water. Indeed, the active revolution of the unhatched 

 animal, whilst closely invested by the egg-membrane, may be 

 of the nature of a geotropic response, persistent because 

 ineffectual. We make use of the negative geotropism of the 

 larv« in collecting them for experiments. 



The downward movement called forth by vibration takes 

 place both in adult and larval Convoluta, but by different 

 methods. The adult generally releases its hold and allows 

 itself to fall to the bottom of the vessel, — sometimes, however, 

 gliding vertically downwards along the vessel's wall. The 

 larvae turn head downwards and swim in a descending spiral 

 to the bottom. 



The positive geotropism of Convoluta is associated with 

 the presence of an otocyst. The statical and equilibrating 

 functions of this organ have been the subject of much recent 

 research, but the influence which it exercises on geotropic 

 response has, we believe, not been determined. That the 

 otolith plays such a part is clear from the fact that occasional 

 larvae of Convoluta do not exhibit the usual positive 

 geotropism when stimulated by vibration. In such cases 

 examination has shown that the otolith was missing, a feature 

 occurring sporadically even in embryos developed from the 

 same clutch of eggs. 



Though the geotropic movement of old and young Convo- 

 luta are among the most striking movements which they ex- 

 hibit, there are nevertheless times when the animals, young and 

 old, are refractoi-y to the stimulus of gravitj*. Thus every day 

 the animals pass, late in the afternoon, into a singular lethargic 

 state. In this condition, observable alike in the open and in 

 the laboratory, the animals lie in inert masses, sticking to one 



