NO. 1846. ON CERTAIN ELEUTHEROZOIC PELMATOZOA—KIRK. 29 



changes to be observed in the evolution of the group all tend in one 

 direction— toward the perfection of the type for a prostrate, eleu- 

 therozoic life. 



There is one structural feature of considerable importance con- 

 cerning which we have insufficient information. That is the location 

 of the anal opening. According to all writers except Haeckel (1896) 

 this opening is adoral in position. Haeckel in Placocystis locates it 

 on the ventral surface of the animal at the junction of the column 

 with the theca. As a matter of fact, considering the high degree 

 of specialization to be found in this group, one would naturally 

 expect an adcolumnal location for the opening, which would only be 

 consistent with modifications to be observed in the case of other 

 prostrate, actively moving Cystidea. If the anal opening in Trocho- 

 cystis actually be adoral, then I feel that we may well postulate the 

 same position for it in the case of the later forms, however. It is 

 quite inconceivable that between TrocJiocystis and Mitrocystis this 

 opening should have shifted from the distal portion of the theca to 

 its extreme proximal extremity. 



General discussion of the Cystidea of Group I. — Consideration of the 

 facts as here presented in regard to the stalked Cystidea indicates 

 that an eleutherozoic habit was maintained among these forms from 

 the earliest times down to their latest appearance. Indeed, it will 

 be noted that the last cystid of which we h^ve a record, Anomalo- 

 cystis disparilis, of the Middle Devonian, is the most highly specialized 

 of all the eleutherozoic Cystidea. The list of genera given above 

 might be increased appreciably. A sufficient number of forms have 

 been cited, however, to indicate the widespread acquisition of this 

 form of life among the most diverse groups of the Cystidea. We 

 know, indeed, that a few stalked cystids, particularly in the Silurian 

 and Devonian, were permanently affixed by cementation of the distal 

 portion of their columns. The majority of the cystids, I think we 

 may be safe in assuming, were detached at least for the greater part 

 of their existence, however. 



This almost universal freedom among the cystids is of particular 

 interest as apparently in no wise impairing the acquisition of sym- 

 metry aiid structures supposed to characterize a statozoic existence. 

 The reason for this is no doubt that in most cases freedom among 

 these forms merely represents non-attachment. For the greater part 

 of their lives the animals, though free, were but slightly less sessile 

 than cemented or rooted forms. The obvious advantages and, indeed, 

 the necessity for postlarval free stages and their bearing on the 

 ecology of the organisms is much the same as that among the Cri- 

 noidea. A discussion of this phase of the question is given in the 

 case of crinoids, and most of the statements there made are equally 

 applicable to the present types. 



