142 BULLETIN OF THE 



ing at fig. 16, PI. VII., if the stellate bodies iu it are spines, and the 

 reticulated network plates of the test of Echinarachnius, and if this 

 genus resembles that described by A. Agassiz in the way the spines are 

 formed, that the stellate cells must have arisen from the reticulations, 

 and been constricted from them. There is nothing to show that in 

 Echinarachnius stellate and reticulated rods arise one from the other ; 

 for these two centres of calcification are distinct in early stages, and we 

 sometimes have stellate rods without the corresponding lace-work rods. 

 This then would throw a doubt on the interpretation which I have 

 given to the stellate calcification as immature spines, unless Echinar- 

 achnius is very different* from the sea-urchins described in Agassiz's 

 account, as far as the growth of the first-formed spines is concerned. 



The question whether the stellate bodies are spines or pedicellariee 

 is a very difficult one. If the spines of the genus Echinarachnius form, 

 like those of Strongylocentrotus, from the reticulated plate of the test, 

 as recorded by A. Agassiz, we cannot regard the stellate bodies as 

 true spines. According to Metschnikoff,t the pedicellaria is one of the 

 first structures to appear in the echinus of E. lividus. PI. VII. fig. 7, of 

 the last-mentioned work, shows a young pedicellaria, which has a very 

 close likeness to some of the stellate calcareous bodies of the young 

 Echinarachnius. The growth of the stellate calcareous body was not 

 traced into a pedicellaria in Echinarachnius. The homology of the five 

 plates surrounding the central plate in PI. VII. fig. 16, has not been satis- 

 factorily made out. It may be conjectured that they correspond with 

 either the genital or ocular plates of the adult, but they were not traced 

 to these plates, and such an interpretation would be conjectural. The 

 formation of new plates, according to A. Agassiz, takes place in Strongy- 

 locentrotus in a spiral manner. The new plates of Echinarachnius are 

 thought to form in the same way as those of other Echinoids, but I 

 have been unable to trace them, on account of the great deposit of pig- 

 ment, and the consequent opacity of the forming test. In stages older 

 than PI. VII. fig. 16, the sea-urchin was nearly opaque. In fig. 17, 

 PI. VI I.J the different plates which compose the test could not be 



* The formation of the stellate bodies which have been identified as spines in 

 Echinarachnius resemble in their growth the growth of the spines of Asterina, as 

 figured by Ludwig (Zeit.f. Wiss. Zool. XXXVIL pp. 67-70, fig. 100), more closely 

 than they do those of Asteroidea and Echinoidea figured and described by Agassiz. 

 Ludwig makes no mention of A. Agassiz's accounts of the development of the 

 spines in starfishes and sea-urchins. 



t Op. cit., PI. Vn. fig. 6. 



