370 Dr. O. Hamann on the Phylogeny 



the form of an oval vesicle projecting into a vacant space 

 (schizocoele-space) might equally well be drawn from a Star- 

 fish. 



In Astericla a follicular epithelium could be demonstrated. 

 In Echinida the first foundation of the ovicell from epithelial 

 cells is so far the same that here also a commencement of 

 follicle-formation is made. It goes no further, howe^^er, and 

 the developed Echinidan ovum possesses a resistant enve- 

 lope, which, however, has been formed from the ovicell and 

 not from a follicular epithelium. 



An apparently great difference in the organism of the Sea- 

 urchin is constituted bj the presence of a special masticatory- 

 apparatus, the lantern. That this is produced by alteration 

 and transformation from whorls of the Starfish seems to be 

 probable. But that no Starfish can be produced from a Sea- 

 urchin provided with a masticatory apparatus is shown with 

 certainty, in my opinion, by the position of the oral blood- 

 lacunar ring and of the water- vascular annular canal. The 

 peculiar positions of these organs in Echinida are in relation 

 to their simpler and more easily intelligible position in the 

 Starfish under the condition of the changed position of the 

 whorls. 



A further important agreement is shown by the water- 

 vascular system. The stone-canal of the Asterida is of very 

 complex structure and forms a smooth-walled tube only in 

 youth. Later on spiral convolutions of various forms appear 

 in the lumen. In the Echinida the canal remains a smooth 

 tube and shows no organization indicative of the structures 

 occurring in Starfishes. This retrogression, as I conceive the 

 simplicity of this organ in the Echinida to be, is closely con- 

 nected with the mode of life of these animals. Their move- 

 ment is in most cases limited in extent. The sucking-feet 

 are only moderately effective, owing to the long spines, and 

 locomotion takes place usually with the aid of the spines 

 employed as stilts. Through this a retrogression of the lon- 

 gitudinal canals (ambulacral vessels) of the aquiferous vessels 

 has taken place ; the ampullse are less developed, and the 

 valves occurring in Asterida have disappeared. In their 

 place the transversely- stretched muscular threads provide for 

 the closure of the ampullae, but of course only in a very 

 insufficient manner. Most of the ambulacral feet are there- 

 fore but little developed, and this applies particularly and in 

 a still greater degree to the Spatangidse, in which the retro- 

 gression has gone much further. 



In two or three words I must refer to the disappearance of 

 the musculature of the body-wall in the Echinida. In the 



