I 86 CASWELL GRAVE. 



immediately surrounding it failed to yield a sufficient supply of 

 food. The tentacles in reaching about over the bottom in search 

 of more, detached the animal and a crawling habit was developed. 

 As the tentacles grew in length and complexity a like develop- 

 ment in the organs which nourish, enervate, support and protect 

 them would naturally follow. The tentacles being five in number, 

 we have in them a possible origin for the pentamerous symmetry 

 which characterizes the nervous and skeletal systems and a con- 

 siderable part of the ccelomic cavities of all echinoderms. At 

 the time when fixed life was given up by the ancestor of those 

 echinoderms which are at present free living, each of its radii 

 probably contained a five-branched tentacle, since this is the 

 number which is possessed by many echinoderms at the period 

 when their metamorphosis is being completed. The period of 

 fixation was long enough and the changes which took place in the 

 organization of the animal at this time were so great that all trace 

 of an anterior or a posterior part, as such, was lost and now, in its 

 second period of free life, the direction of locomotion depends 

 wholly upon external conditions. 



During the period when the common ancestor of the group 

 was fixed, differentiations into at least three different types took 

 place. One line is now represented by holothurians, one by 

 crinoids and another by asterids, echinoids and ophiuroids. 

 Among crinoids alone the fixed condition has been retained. In 

 this group the problem of enlarging its base of supplies was solved 

 not by becoming free but by the elongation of the organ of attach- 

 ment, and by the migration of the mouth and tentacles still further 

 toward the opposite end. In the type which has given rise to 

 holothurians, the mouth and tentacles migrated in just the oppo- 

 site direction, viz., into the organ of attachment and were thereby 

 brought into relation with the bottom. The free-crawling habit 

 was later acquired. The ancestor of the starfishes and sea- 

 urchins made no permanent use of its organ of attachment and 

 no further migration of the mouth took place but it was brought 

 into direct relation to the bottom by the rotation of the body as 

 a whole. 



ZOOLOGICAL LABORATORY OF THE JOHNS HOPKINS UNIVERSITY, 

 April, 1903. 



