ECHINOIDEA. II. 



ambulacral plates are found to be pushing out from the plates of the anal system on each side of the 

 genital plates. As the ocular and genital plates of the genital ring become separated, with increasing 

 size, the additional anal plates forming in the intervening spaces are pushed out, and become a part 

 of the abactinal portion of the interambulacral area. To this I remarked (p. 175): This statement is 

 completely incorrect. The interambulacral plates are formed in Ph. placenta as in other Echinids, not 

 by the anal plates. The genital ring, at all events, is closed, until the animal has reached a size of 

 I7 mm in diameter, and so far accordingly the interambulacral plates must necessarily be formed in the 

 common way, as may also easily be substantiated. In a specimen of a diameter of 30""" a couple of 

 ocular and genital plates are still joining, and here the case is quite the same. That a new mode of 

 formation of the interambulacral plates, otherwise quite unknown among- the Echinids, should then 

 suddenly occur, is very improbable and, above all, Agassi z has not at all proved it; all that may 

 be seen in the larger specimens, is that the small anal plates directly adjoin the uppermost interam- 

 bulacral plates. - I am quite unable to find in this criticism any accusation of ignorance of em- 

 bryological facts, and I am unable to see, likewise, that 1 am mistaken in my criticism. So far 

 as I can understand the meaning of the above passage quoted from the Blake-Echini, Professor 

 Agassiz maintains here that the anal plates are directly developed into interambulacral plates. That 

 mode of development would be in direct opposition to the generally accepted views on the homology 

 of the Echinoid-skeleton, which hold that the interambulacral plates and the anal plates are of very 

 different morphological value. A transformation of the anal plates into interambulacral plates is thus 

 very improbable for morphological reasons, further also, on account of the younger specimens showing the 

 normal condition, and finally, I must repeat that Professor Agassiz has not shown it to be the case. 

 On examining Mr. Westergren's admirable figures of the abactinal system of Phormosoma liispi- 

 dHin on PI. 39 or of Asf/ienosoma corlaceiim on PI. 52. fig. i of The Panamic Deep-Sea Echini , it is 

 easily seen that the young interambulacral plates originate at the sides of the ocular plates and are not 

 transformed anal plates. That the anal plates push their way down into the median part of the inter- 

 ambulacrum, separating the two series of interambulacral plates at their upper end, I have never denied 

 or thought absurd; but I must maintain that these anal plates never become interambulacral plates, which 

 was, so far as I am able to see, the meaning of the statement given in the Blake -Echini. Whether that 

 is also the meaning maintained in the Panamic Deep-Sea Echini. I am unable to gather; the ex- 

 pression a flow of the anal plates into the interambulacrum similar to the flow of the ambulacral 

 plates of the corona on to the buccal plates of the actinal systems, as well as the expression the intru- 

 sion or flow of the anal plates into the interambulacral system > (p. 117) do not seem to mean a trans- 

 formation of these plates into interambulacral plates. If that be the case, Professor Agassiz seems to 

 me to put a new meaning into his old statement, and thus, his remarks against my criticism have no 

 bearing against me, since I have never thought of saying a word against the latter meaning. 



I think I have now answered all the criticisms which Professor Agassiz directed against 

 me. There are only a few of his more general remarks on the Echinothurids concerning which I must 

 say a few words on this occasion. 



Professor Agassiz begins the Chapter on the Echinothurida with this remark: We may be 

 justified in assuming that the anal system is in the Echinothuridae, as in the Cidaridae, covered by 

 five small anal plates > (p. 75). I do not think we are justified in making this assumption. The youngest 



