OF ARTS AND SCIENCES : APRIL 14, 1863. 109 



The open pentagons do not close till after the star-fish has absorbed 

 the whole of the larva. The complicated system of arms and the 

 whole of the Brachiolaria is absorbed by the star-fish, which is not 

 separated from the larval stock, as seems to be the case in Bipinnaria 

 according to Mliller's statements. 



The arms of the star-fish are broad and short in the young. (See 

 Figs. 14, 15, 16.) The suckers are pointed, and arranged only in two 

 rows (Fig. 17). The disc is developed only later. The odd terminal 

 tentacle has an eye at its base, and never develops a disc (Fig. 17). 

 The abactinal surface is very arched (Fig. 18). The spines are arranged 

 in regular rows, and the plates remind us of the arrangement of plates of 

 Crinoids (Fig. 16). The anus opens near the edge of the disc on the 

 lower side. The madreporic body also is situated on the edge (Fig. 17). 



The mode of development of star-fishes, as observed in our Astera- 

 canthion, cannot be called a case of alternate generation, nor is it a 

 metamorphosis in the ordinarily received sense. It is, in fact, a mode of 

 development peculiar to Echinoderms, something entirely different from 

 what we find in any other class of Radiates. It is not an alternate 

 generation ; for the Brachiolaria can in no way be called a nurse, as 

 each Brachiolaria produces but one star-fish, the whole Brachiolaria 

 being absorbed by the star-fish, and not a single appendage left out. 

 Nor is it a metamorphosis, as the changes which take place are so 

 gradual, that at no time can the line of demarcation be drawn between 

 two stages, with any degree of precision, as in Crustacea or Insects. It 

 is a mode of development eminently Echinodermoid, and whether we 

 observe it in the Ophiurans, the Sea-urchins, the Holothurians, or the 

 Crinoids, there seems no doubt, from the observations of Miiller, that 

 it takes place according to one and the same plan. 



Lately there has been a great deal of discussion among the writers 

 on Echinoderms, as to whether the madreporic body was a proper 

 point to start from to draw the axes of the body ; Agassiz, on one side, 

 maintaining that the madreporic body was constantly in the same rela- 

 tion to the different parts of the Echinoderm ; while Miiller, Desor, and 

 Cotteau have been the most prominent opponents of this view. The 

 mode of formation of the madreporic body seems to me to decide this 

 question. It is invariably on the left water-tube that we find the 

 madreporic body. While the star-fish is developing, it is placed at the 

 angle of the upper arm, and the natural consequence is, that the madre- 

 poric body will invariably be found opposite the middle arm of the 

 VOL. VI. 12 



