and Anatomy of the Echtnodermata. 371 



Starfishes I have demonstrated in tlie body-wall of each arm 

 an annular and a longitudinal muscular layer, such as exist 

 in the same way in the Vermes. In the Echinida the rays 

 (the arms) are amalgamated with the body, the calcareous 

 secretions form a skeleton consisting of ten pairs of plates, 

 for which muscles in the body- wall have become unnecessary. 

 If, then, we assume that the Holothuria3 have branched off 

 from the Echinida, this must have occurred early, that is to 

 say they must have originated from forms in which the 

 musculature had not yet retrograded nor the skeleton been 

 developed, as is the case in existing Echinida. According to 

 Ludwig's * discovery in Spatangidte, on the dorsal surface 

 between the rows of plates situated above the periproct there 

 are muscular fibres at the point where they meet in the middle 

 line. This musculature, which consists of short (1 millini. 

 long), smooth, muscular fibres, notched at their extremities, 

 is to be regarded as the remnant of the annular (and longitu- 

 dinal) musculature of the body-wall, such as is shown by the 

 Starfishes. 



3. What Structures are we to regard as Sanguiferous 

 Spaces in the EcMnodermata ? 



The older naturalists supposed that in the Asterida the 

 five or more longitudinal canals running in the ventral sur- 

 face of the arms were the blood-vessels, and that the annular 

 cavity surrounding the oesophagus, which unites these five 

 or more canals, was the annular vessel. It was shown, 

 however, by Lange and Teusch, that these radial or 

 ambulacral longitudinal canals were divided in their whole 

 length by a vertical band, and that this band in its whole 

 extent was traversed by interstices and cavities. In the latter 

 they recognized the true blood-vessels, or rather blood-lacunae. 

 That the conditions are the same in the dorsal body-wall, 

 and that here also the true blood-lacunce (the anal ring of 

 blood-lacunse and the lacunae leading to the sexual organs) 

 lie in such canals, has been shown by Ludwig, who proposes 

 the name of perihcemal canals for the latter. At the same 

 time, however, that naturalist supposed that the periheemal 

 canals were in connexion with the body- cavity, the entero- 

 coele. I have shown, by demonstrating the origin of these 

 canals as also of the ventral blood-lacunae, that perihEemal 

 cavities as well as blood-lacunae of the septa or suspensory 

 bands are schizocoele-formations and therefore homologous 

 structures. This applies also to the cavitary system dis- 



* " Ueber bewegliche Schalenplatten bei Ecbinoideen/' in Zeitschr. 

 fur wiss. Zool. Bd. xxix. 



26* 



