18 



ON THE EMBRYOLOGY OF ECHINODERMS. 



after the manner of Asteracanthion Mulleri of Sars, without passing through the plu- 

 tean stage. So many points of investigation have arisen since those observations were 

 made that they must necessarily be very incomplete without a renewed comparative 

 study of the whole subject. I here copy from the drawings of Professor Agassiz two 



stages of the young Ophiopholis which will enable us to compare the two modes of 

 development, and show us as clearly for Ophiurans as I have shown for Asteracan- 

 thion in the fifth volume of Professor Agassiz's Contributions, that these two modes of 

 development are but longer or shorter ways of arriving at the same point. Fig. 29 is 

 one of the more advanced young of Ophiopholis seen from the abactinal side. At this 

 stage it shows prominently the arrangement of the abactinal plates and the position of 

 the arm-joints. As in young Asteracanthion, we have a central plate, and five radial 

 and five interradial plates. No hooks or granules are as yet developed ; only two of the 

 tentacles have reached any great development, and project beyond the edge of the arms. 

 A careful examination of the young Ophiura in this stage would enable us to deter- 



mine the exact place where new arm-joints are added. There seems some doubt, from 

 the observations of Muller* and of Lutken,-f as to whether the new joints are formed 

 at the base of the arms or at the extremity. From what I have observed in Starfishes, 

 it is evident that the new parts of the actinal and abactinal portion of the arms are not 

 added at the same place. The new suckers in the Starfishes are formed nearest to the 

 odd terminal ocular tentacle, while the new spines of the abactinal side of the arm 

 are formed at the base of the arm. (See Vol. V. of the Contributions of Professor 

 Agassiz, where a full account of the increase of the arms of the young Starfish will 

 be found.) Something similar may occur in Ophiuridae, and would account for the 

 difference of opinion entertained by Muller at different times as to where the new arm- 

 joints were added. In fig. 30 we have the actinal side of a young Ophiopholis some- 

 what less advanced than fig. 29 ; there are as yet no arm-joints, and the whole outline 



is pentagonal ; two of the tentacles alone make their way through the actinal lime- 

 stone floor. 



Amphiura squamata Sars. 



The Pluteus of which a figure is here given (fig. 31) is probably the larva of 



mers 



Amphiura squamata. I have found these lame only three times during two sum 



once in June, once in July, and then the young Ophiuran (figs. 32 and 33) 

 during the first week of October. It probably requires as long a time as this for the 

 development of the Echinoderm, as I have kept the larva which were caught in June 



* 



t 



Muller, J. Memo 



ad Historiam 



Kjobenhavn, 1858 



