Cdap.II. FORMATION OF THE AMBULACRAL SYSTEM. 29 



stomach, or any part of the alimentary canal, as Miiller believed (PL V. Figs. 1, 8; 

 PI. III. Figs. 1-11), but that a kind of cap has been formed round it by the water- 

 tabes. Owing, however, to the accumulation of very fine granules of limestone, the 

 anal extremity has by this time lost its transparency ; this would be easily mistaken 

 for an encroachment on the stomach itself. In proportion as the abactinal region 

 becomes solidified (PL III. Fig. 11 ; PL IV. Figs. 1, 2 ; PL VII. Fig. 8), the stomach 

 loses its globular shape, and becomes from this time forward flattened and pear- 

 shaped. Previously to the formation of the Starfish on the surface of the two 

 water-tubes, placed on opposite sides of the stomach, we could trace no change of 

 form in the stomach itself. From the time, however, when the Starfish encroaches 

 little by little upon the anal extremity of the larvae, it pushes slightly to one side 

 the stomach and the intestine, owing to the great increase in bulk of its actinal 

 and abactinal areas. The anal portion of the water-tubes now swells and contracts 

 in such a way as apparently to divide that portion of the water-tubes into chambers; 

 but, on watching the circulation of the fluid in the water-tubes, for any length of 

 time, the currents can be followed flowing from one of these elliptic chambers to 

 the other, plainly showing the different planes in which the ventral and dorsal part 

 of the tubes are placed to be the only cause of this delusion. 



Miiller has distinctly stated, over and over again, during the course of his inves- 

 tigations, that the young Echinoderm was formed by encroaching upon the stomach 

 itself ; I am satisfied, from repeated observations of this point, in Starfish, Sea-urchin, 

 and Ophiuran larvse, that this is not the case. The mistake arises from the fact 

 that the water-tubes, by their extension and increase, cover and conceal part of the 

 stomach, forming a sort of hood over it ; the two sides of the young Echinoderm, 

 separated by the whole width of the stomach and the thickness of the two water- 

 tubes, forming upon the outer surface of the latter, and not in any way encroaching 

 upon the stomach, which is simply enclosed by the actinal and abactinal areas of 

 the Echinoderm. Had I not traced this with the greatest care, I should scarcely 

 venture to doubt the statements of Miiller, but I am satisfied that he was mistaken 

 in this explanation of the mode of the formation of the Echinoderm. 1 



Formation of the ambulacral System. — "We have already seen that the very first 

 changes which take place in the water-system (w, w) consist of the five folds (t, PL 

 V- Fig. 2) extending obliquely across the exterior surface of one of the water-tubes 

 (w). From the fact that these folds develope across the surface of an elliptical 

 tube, the five folds naturally form a twisted spiral, with pentagonal outline each 

 side of this spiral, forming the first nucleus of the five ambulacral tubes. I speak 



1 It may not be out of place to say, that Profes- of the accuracy of every point which seemed in the 

 eor Agassiz, during this investigation, satisfied himself least contradictory to the statements of Muller. 



