284 C. M. CHILD. 



able that the mesenteries of these parts are so closely pressed 

 together that the mesenterial chambers are almost obliterated 

 and their communication with the general enteric cavity almost 

 or quite shut off. After the other parts have once become dis- 

 tended these collapsed parts are compressed. Only as water 

 gradually finds its way into them, either by diffusion or through 

 small openings, will they become distended so as to permit the 

 typical circulation. 



A case obtained in a similar manner from a piece of C. incui- 

 branacens is shown in Fig. 7 (natural size). It could be seen 

 without difficulty that the three regions bearing the long ten- 

 tacles were distended while the intermediate regions were not or 

 were only slightly. In the course of a few weeks the distension 

 of all parts gradually become uniform and the de- 

 layed tentacles finally attained the same size as the 

 others, though only after the others had ceased to 

 elongate. Of course at a somewhat earlier stage 

 than that figured not even minute tentacle-buds were 

 present on the infolded regions though the tentacles on 

 the distended regions were already well developed. In this case as in 

 the others it is probable that in the infolded regions the mesenteries 

 are so closely pressed together that little water enters the chambers. 

 These experiments have a certain bearing upon the question of 

 localized pressure as a factor in determining the position of the 

 marginal tentacles. It is evident that in the distended portions 

 the circulatory currents must possess greater force and volume 

 than in the collapsed portions, where they may indeed be absent 

 or insufficient to act as stimuli. If the appearance of the mar- 

 ginal tentacles is due to the local pressure resulting from these 

 currents it is easy to see why the tentacles appear earlier on the 

 distended portion. In general internal pressure we can find no 

 factor which can determine the position of these organs, and if 

 they are determined by "protoplasmic" factors it is difficult to 

 understand why they should appear earlier on the distended por- 

 tions than on the others. 



SUMMARY. 



i. Except when the animal is contracted or collapsed the 

 body-wall of CcriantJins is subjected to a certain degree of ten- 



