04 EMBRYOLOGY OF THE STAEFISH. 



body in such cases be owing to the continuance of this embryonic char- 

 acter?— the natural result of which would be, to throw the madreporic 

 body slightly on one side of the middle line, so that, though still retain- 

 ing its position opposite the third arm, an axis passing through them 

 both would not divide the spherosome into symmetrical portions. If 

 there were in nature such forms as asymmetrical Starfishes, analogous to 

 the Echinometrada3, they Avould be represented by the embryonic Star- 

 fishes of PI. VI. Fifjs. 1-G, in whicli a line drawn through the madre- 

 poric body and the middle of the odd arm would not divide the Starfish 

 into symmetrical halves. Suppose the flattening of the young to be 

 completed without the loss of this want of sj-mmetry, and we have a 

 form representing Echinometra-Starfishes, if any such exist in nature. 

 The fact that in some of these Echinometrada) the axis, passing through 

 the madreporic body and this long arm, crosses the median line from 

 opposite sides, could be easily explained on the supposition that the 

 former is placed on the ventral instead of the dorsal side of the larva. — 

 an assumption which is not unfounded, as this occurs in Ophiurans and 

 in young Starfishes. In this way the change of position in the direction 

 of the axis whicli is found in Acrocladia and Podophora on one side, 

 and in Echinometra on the other, could be easily explained. [For fuller 

 discussion of the bearing of the positions of the madreporic body deter- 

 mining the anterior axis of the Echinoderms, see my Revision of the 

 pA'hini, Part lY., and the description of Salenia. Consult also the Memoir of 

 Loven (Etudes sur les Echinoidees).] In Echinoids the actinal and abac- 

 tinal areas are formed upon the exterior surfaces of the water-tubes, as 

 in Starfish larva?. This I have shown in the paper referred to above, 

 published in the Memoirs of the American Academy for 1864. The 

 earlier a])pearance of the tentacular pentagon in Echinoids and in Ophiu- 

 rans is that of a spiral on the surface of the water-tubes, similar in plan 

 to that observed in our Starfish larva; it is evident that the additional 

 plates formed in a ^oung Sea-urchin arise spirally, and from what is 

 known of the mode of formation of the young Echinus and 3'oung Ophi- 

 \w\\n, it follows, necessarily, that the ambulacral system in both must 

 have been oi)cu pentagons, becoming connected only by the closing of 

 the surfjices upon which the 3'oung Sea-urchin or Ophiuran wore de- 

 volojx'd. 



An exaniiuutiou of the (igures of our young Starfish, just after the re 



