BIPINNAEIA ASTEEIGEEA. 69 



the back, is open ; and although he has occasionally represented it as 

 such, he has not perceived the true relation between the positions of 

 these two areas. He says distinctly that the cloak-like envelope, or the 

 abactinal area, originates upon the surface of the stomach, whereas it lies, 

 in reality, upon the surface of the second water-tube, which he says does 

 not exist in his Bipinnaria ; while the water-system, or the ambulacral 

 system, originates on the water-tube in such a way that the two open 

 Avarped pentagonal surfoces, the actinal and the abactinal areas, make a 

 very large angle with one another; Miiller, however, did not notice that 

 they were open and warped surfaces. 



Van Beneden's observations, in which he says that the two branches 

 of the Y-shaped water-tubes are separate in the young, and become united 

 in the adult, are fully confirmed by my observations. Miiller has called 

 these small bodies, while they are still separate, problematic bodies ; he 

 says they disappear in older larvae, and have nothing to do with the 

 " Schlauch-System." It is evident, from my observations, that the Schlauch- 

 System is only the advanced condition of tlie problematic bodies, which 

 are isolated on each side of the body in the young larvte (see Pis. II., 

 III. of this Memoir, and Van Beneden's Brachina), and become united 

 in a Y-shaped water-system (Schlauch-System), when they reach the con- 

 dition of Bipinnaria of Miiller. It wo\ild seem, from his figures, as if 

 the abactinal pentagon closed, while the Bipinnaria is still visible. I am 

 rather inclined to think that more advanced larvaj will be found to be 

 Brachiolaria-like, as is the case with our Starfish and the Brachiolaria 

 from Messina ; and that this apparent closing up is due to the fact that 

 the larva is not in its normal state, or that the drawings are made some- 

 what foreshortened. In the second Memoir of Miiller, on Plate I., we 

 see that the Y-shaped water-system (Schlauch-System) has been noticed 

 in two of the larva? {Figs. 4, 7), while in the intermediate stages, and 

 in younger larva\ it has escaped his notice. It is midoubtedly to Miiller's 

 want of acquaintance with the earlier and later stages of his Bipinnaria 

 that we must ascribe the discrepancies in his observations. Many of the 

 more important points in the structure of the young larvaj naturally es- 

 caped Derbes and Krohn, who were not familiar with the adult larvte ; 

 neither of these observers tells us anything of the presence of the water- 

 tubes, or of the first appearance of the young Echinoderm. 



Bipinnana asterigera. — Miiller's views concei-ning the different organs of 



