ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 63 



vesicles ; the superior of these is the hydrocoel, the inferior the entero- 

 coel. The latter becomes a band-like body, which gives rise to the two 

 sacs of the ccelom. The fine canal which puts the interior of the 

 hydrocoel-vesicle into connection with the outer world by means of the 

 dorsal pore is the primary stone-canal of Holothurians ; this canal lies 

 interradially to the five primary tentacles ; while these primary tentacles 

 are, in all Echinoderms, radial in position, the secondary outgrowths of 

 Holothurians are interradial. 



The elements of the mesenchym do not only form a subcuticular 

 layer, but they give rise to unilaminate investments for the enteron, 

 ciliated bands, and nerve-bands, to stellate cells in the gelatinous sub- 

 stance, to the muscular elements, and, lastly, are the seat of origin of 

 the calcareous deposits. Physiologically the calcareous concretions 

 appear to be of importance for the larva, as they make the lower part of 

 the body heavier than the upper, so that the animal always moves in 

 water with the lower end more or less directed downwards. The 

 rudiments of the calcareous ring appear while the rosette-like rudiment 

 of the water-vascular system lies freely in the gelatinous substance to 

 the left of the fore-gut. These are, at first, merely fine delicate rods, 

 which occupy an interradial position. 



In the passage of the Auricularia to the tun-shaped form the most 

 remarkable phenomenon is the hitherto unnoticed diminution in the 

 length of the various axes ; while a fully developed Auricularia has a 

 long diameter of 1 • 4:-to 1 • 7 mm., the larva in the pupal stage and the 

 quite young Synapta (^Pentadula) is only from • 4 to 0*5 mm. long. 



With this diminution in size there is a loss of transparency, owing 

 to the closer approximation of the mesenchym-cells. 



The tun-shaped larva with ciliated bands, its conversion into the 

 young Synapta, the young and adult Synapta, are described at a length 

 greater than that which we can follow. 



In the second half of his memoir Dr. Semon deals with the phylogeny 

 of the Echinodermata. He commences by raising the question of the 

 position of the Synaptidse among the Holothurioidea ; as to this, he 

 concludes that there are no facts of structure and development which 

 justify us in supposing that the simple organization of the Synaptidae is 

 due to reduction from the more complicated organization of the pedate 

 Holothurians. Secondly, as to the relation of the Holothurians to 

 other classes of the Echinodermata. The former are all distinguished 

 by the fundamental peculiarity that their body water-vessels lie ad- 

 radially and not radially, for these vessels arise from the secondary 

 interradial evaginations of the water-tube. In all other Echinoderms 

 the madreporic plate lies interradially to the rays of the primary 

 tentacles ; it is perradial in Holothurians, on the supposition that the 

 secondary evaginations are the homologues of the primary tentacles of 

 other Echinoderms. But this is a viev we can hardly accept, and we 

 must rather suppose that the primary tentacles of Holothurians are 

 comparable to the primary tentacles of other Echinoderms, and that the 

 secondary evaginations are special formations, Goette alone has per- 

 ceived that the body ambulacra of Holothurians correspond to the 

 interradii of the star-fish. If this view of homologous parts be true, it 

 follows that it is quite impossible to suppose that the Holothurians were 

 developed from echinoid-like forms, and we must rather suppose that 

 the two groups separated before a water-vascular system was developed, 



