C. M. CHILD. 



Another marked though gradual change which occurs is in the 

 longitudinal retractor muscles. Contraction and invagination of 

 the oral end is usually rapid and considerable in amount in ani- 

 mals newly removed from their burrows, but as the length of the 

 body decreases this reaction becomes less and less marked. 

 Animals in the condition of Figs. 11 and 12 are usually inca- 

 pable of complete invagination of the disc, no matter how powerful 

 the stimulus. Usually invagination does not include more than 

 the basal half or perhaps two thirds of the tentacles in such cases, 

 though originally the disc and tentacles could be withdrawn far 

 into the body. 



Two factors are probably concerned in this decrease of the 

 power of invagination of the oral end : in the first place, the re- 

 tractor muscles are undoubtedly undergoing atrophy from disuse, 

 since as the body decreases in length their functional activity is 

 much reduced ; and secondly, the enteron is much smaller than 

 it was originally and, as can readily be shown by opening one of 

 these reduced specimens, is almost wholly occupied by the mes- 

 enteries with their muscles and filaments, and the gonads, if they 

 are present, leaving but little space for water. Consequently 

 when contraction occurs, decrease in size can occur only to the 

 extent that water is expelled. In these specimens, after the ex- 

 pulsion of the small amount of water present, the body is essen- 

 tially a sac packed full of tissues and no further contraction is 

 possible unless parts of these are extruded, which does not com- 

 monly occur in Harcnactis. 



Some of the different individual shapes after four and a half 

 months of life outside of a burrow are shown diagrammatically in 

 Figs. 1316. Fig. 13 represents a type very common in my ex- 

 periments. These specimens have apparently lost all power to 

 attach themselves and the aboral end shows no trace of a distinct 

 foot-region. The whole body is merely a rounded sac with but 

 little power of elongation or of contraction, and invagination of 

 the oral end does not proceed beyond a slightly decreased diam- 

 eter and increased concavity of the disc. Animals of this shape 

 occur chiefly among those whose attachment to the bottom of the 

 dish has often been disturbed, c. g., by changing the water or by 

 examination for other purposes. I am inclined to believe that 



