52 EMBRYOLOGY OF THE STARFISH. Part I. 



odd arm. The case of the Echinornetradgg and Salenidse is constantly quoted to show 

 that the madreporic body is not connected with any definite axis. But might it not 

 be that a stage, which is embryonic in the young Starfish — viz. : that preceding 

 the closing of the actinal and abactinal areas — is probably retained in those Echinoid 

 families in which the process of closing is not completed? And may not the unsyni- 

 metrical position of the madreporic body in such cases be owing to the continuance of 

 this embryonic character ? — the natural result of which would be, to throw the madre- 

 poric body slightly on one side of the middle line, so that, though still retaining 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 EchinometradaB, they would be represented 

 by the embryonic Starfishes of PL VI. Figs. 1-6, in which a line, drawn through the 

 madreporic 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 symmetry, and we have a form representing Echinometra - 

 Starfishes, if any such exist in nature. The fact that, in some of these Echinome- 

 tradas, 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 r the laifva, 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 which 

 is found in Acrocladia and Podophora on one side, and in Echinometra on the other, 

 could be easily explained. In Echinoids, the actinal and abactinal areas are formed 

 upon the exterior surfaces of the water-tubes, as in Starfish larvce. This I have 

 shown in the paper referred to above, published in the Memoirs of the American 

 Academy for 1864. The earlier appearance of the tentacular pentagon in Echi- 

 noids and in Ophiurans 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 young Sea-urchin arise spirally, and from what is known of the mode of 

 formation of the young Echinus and young Ophiuran, it follows, necessarily, that 

 the ambulacral system in both must have been open pentagons, becoming connected 

 only by the closing of the surfaces upon which the young Sea-urchin' or Ophiuran 

 were developed. 



An examination of the figures of our young Starfish, just after the resorption 

 of the larva (PL VI. Figs. 2, 3, 4), in which a line, drawn from the madreporic 

 body through the middle of the odd arm, would by no means divide the Starfish 

 symmetrically, confirms the above explanation of the eccentricity in Echinometra. 

 Supposing the spiral to have been formed from the other side, the obliquity would 

 be in the opposite direction. Of course this is simply a supposition on my part, 



