Development of Holothurians. 415 



worthiness of the statements made by Selenka as to tlie earliest 

 •developmental stages ot" Cticumaria Planci, I did not begin 

 my investigations nntil the eighth day of the development. 

 Subsequently, however, the conviction has forced itself upon 

 me that my confidence went too far. The stages of the first 

 seven days of development also must be investigated afresh, 

 and I hope that I shall succeed in obtaining these tliis spring. 

 The following observations, therefore, refer exclusively to 

 stages which are older than seven days, 



I was unable to confirm the customary view that in the 

 Holothuria the plane of symmetry of the young Echinodcrm 

 coincides with that of the larva. On the contrary, these two 

 planes intersect one another in the same way as I have proved 

 them to do, e. g. in the development of Asterina gibhosa. In 

 the anterior (oral) region of the stage which is transitional 

 between the barrel-shaped larva and the young Echinodcrm 

 the plane of symmetry of the young animal diverges from 

 that of the larva towards the left, but in the posterior region 

 towards the right. The two planes of symmetry, therefore, 

 cut one another at acute angles. In addition to this, the 

 longitudinal axis also of the young Gucumana is not iden- 

 tical with that of the barrel-shaped larva. In the anterior 

 region of the body the longitudinal axis of the young Cticu- 

 maria diverges towards the ventral surface, in the posterior 

 region, on the contrary, towards the dorsal surface from that 

 of the larva. The peculiar difficulties which beset the proper 

 orientation and the comprehension of transverse and longitu- 

 dinal sections iire evident at once from these conditions. 



Wate7--vascidar System. — The water-vascular ring and the 

 radial canals have already assumed their permanent position 

 on the eighth day. The spot at which the closure of the ring 

 took place can no longer be distinguished. The general 

 position of the water-vascular ring, corresponding to the 

 relations of the plane of symmetry and the longitudnial axis 

 as mentioned above, is such that its ventral region lies farther 

 towards the rear than its dorsal region, and at the same time 

 its left half slightly further towards the rear than its right. 

 It is only loosely connected with the fore-gut by means of 

 few fine, short, suspensory fibrils. I was not able to detect 

 muscle-fibres in the wall of the water-vascular ring at any of 

 the stages which I examined. The five radial vessels arise from 

 the ring with a wide lumen, without any constriction or forma- 

 tion of valves. The median ventral radial vessel on the eighth 

 day already extends backwards with its blind end to a point 

 somewhat beyond the place of origin of the first two sucker- 

 feet canals, wiiich arise from it and are already in existence. 



29* 



