VIII EGHINODEBMATA—PHYLOGENY 551 



difficulty. They could easily have been in the short thick anus, while the terminal 

 portions of their efferent ducts opened outward interradially. 



The Ophiuroidea branched off from the series leading to the Crinoids by the 

 readoption of the free manner of life later than the Echinoids. They used the 

 free arms for locomotion, and took food direct into the mouth. The tentacles never 

 became ambulatory tube-feet, but only retained their respiratory functions. The 

 food grooves closed to form tubes, becoming subepithelial radial nerves with their 

 epineural canals. For further protection, the radial longitudinal rows of ventral 

 shields became arranged over the closing food grooves. The use of the arms almost 

 exclusively as locomotory organs determined their slender form, which makes them 

 appear as mere appendages of the body, and it further led to the return of the 

 gonads into the disc. 



The Asteroidea were the last to branch off from the series of attached Echino- 

 derms with arms, by the adoption of a free manner of life. They used their tube- 

 feet first for locomotion, and secondly for seizing and holding fast prey, which was 

 enveloped en masse direct by the evagiuated oral portion of the intestine and drawn 

 through the mouth into the stomach. The food grooves now no longer serve as 

 such, but are retained as deepened ambulacral furrows, from the bases of which the 

 closely crowded tube-feet rise, and into which they can withdraw. Over the 

 tentacles, Avithdrawn within the furrow, the spines which border the furrow can 

 bend protectively together. Deep in the base of the furrow, the radial nerve ridge 

 is found still in its epithelial position. 



From the standpoint of the foregoing it is to be expected that the ontogeny of 

 the Rolothurioidea, which earliest gave up the attached life, should show the least 

 trace of the phyletic stage of the attached animal, and that by the avoidance of those 

 complicated rearrangements which attachment caused, their development should be 

 much simplified. The facts agree with this expectation, and also with the expecta- 

 tion that an attached stage is most likely to be ontogenetically repeated in the 

 Asteroids (cf. the ontogeny of Astcrina, with its temporary fixation by means of the 

 anterior part of the body, the larval organ). 



In the foregoing attempt to trace the phylogeny of the Echinoderms, we have 

 avoided going into details, and we have also avoided all reference to many 

 important points, such as the hydropore and the stone canal, the hydrociel and the 

 left anterior enterocoel, etc. etc. These can only be elucidated by renewed research, 

 which must be both extensive and thorough. In applying our views to explain 

 special points in Echinoderm morphology, it must be acknowledged that in the 

 majority of cases it does not suffice for a full explanation, and that it cannot indeed 

 at present be reconciled with many ontogenetical and anatomical facts. The recent 

 researches in Echinoderm morphology and the attempts at phylogenetic explanations, 

 which are continually suggesting new points of view, justify us, however, in hoping 

 that, little by little, many of these interesting and important problems will receive 

 a satisfactory solution. 



Review of the most important Literature. 



Comprehensive Works. Text Books. Treatises of Wider Scope. Re- 

 searches extending over some or all of the Classes. 



Agassiz. I'ahcoutological and cmhrijoloyical development. Address hcforc the 

 A mcrican Association for the Advancement of Science. Boston Meeting. Cam- 

 bridge, 1880. Also Ann. and Mag. Nat. Hist. (5). Vol. VI. 1880. 



