NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 3 



eleutherozoic types appeared sporadically, founding, as a rule, short- 

 lived lines that although successful in a small way, had the Pelma- 

 tozoan structure too deeply impressed upon them to permit of any 

 considerable deviation from the parent stock. 



With the possible exception of the Holothurioidea, we may hold, I 

 thinli:, that such eleutherozoic echinoderms as are known to us have 

 been derived from statozoic ancestors. It is but natural that upon 

 the acquisition of a detached or semidetached form of life changes 

 should begin to appear in the structure of the organism. If given 

 sufficient time, these changes so deeply affect the structure of the 

 animal that it is a very difficult matter accurately to establish lines 

 of descent. This is true to a far greater extent than among those 

 forms that remain attached. Among the latter evolution of adult 

 characters is largely orthogenetic in its tendencies, and it is possible 

 to reconstruct the ancestral forms as well as predict the types to 

 come with a fair degree of certainty. In the case of the eleutherozoic 

 forms, however, we have one newly acquired set of tendencies super- 

 imposed upon another set. These secondary tendencies, induced as 

 they are by a form of life widely at varience with that under which 

 the first set operated, tend to vitiate the force of many of the primary 

 tendencies, if not indeed to nullify some of them. In addition certain 

 of these newly acquired tendencies initiate structural changes which 

 diverge widely in their nature from the given line of evolution, and 

 are of such a type as largely to mask and render unintelligible the 

 characters that go to help in determining the genetic affinities of the 

 animal. Such being the case, one's efforts to establish relationships 

 among these aberrant forms are apt to be unsatisfactory at best. In 

 many cases, however, the eleutherozoic Pelmatozoa stand so near 

 the points of inception of their several lines that the problem is not 

 greatly complicated by the presence of altered or superimposed 

 structures. 



DIVISIONS OF THE PELMATOZOA. 



The Pelmatozoa to be discussed may be divided into three main 

 groups : 



I. Those forms retaining jointed columns throughout life, but not 

 using them for permanent attachment. 



II. Those forms which at some stage of development permanently 

 lose all or the greater part of their columns, becoming truly eleu- 

 therozoic. 



III. Those forms that are permanently attached by means of a 

 base of varying composition as regards the constituent elements. 

 No true jointed column is present. The members of this group I 

 shall here style pedunculate forms, for convenience of reference. 



