MUSEUM OF COMPARATIVE ZOOLOGY. 27 



not observed to carry spines, and the interbrachials, in their younger 

 stages at least, are destitute of these structures. 



With the advancing growth of the starfish, the number of spines on 

 the primary plates increases, and new spines are formed on new plates 

 as they appear. The later formed spines, however, never have that 

 prominence so marked in the younger and primary plates, but appear 

 more compact, and more like the spines of the adult Asterias. 



Pedicellarioe. — The pedicellariae, pd, were first observed on the 

 terminal plates in a stage of the starfish in which there were four 

 median dorsals (PI. IV. fig. 3). They were then confined to that 

 plate, being absent on all others. In an older starfish, or one with 

 seven median dorsals (PI. IV. fig. -i), they were likewise found in clus- 

 ters on the second marginal plates, to 2 , and one or two were likewise 

 seen on the plates, to 3 , between the second marginals and the termi- 

 nals. Although in both these stages large spines exist on the median 

 dorsals, d, there are no pedicellariae as yet formed upon or near them. 

 Like the spines, the calcifications of the pedicellarioe are at first wholly 

 separate from the plates from which they rise. Unlike the spines, 

 however, their calcification is from the first double, or split longitudi- 

 nally into two separate parts. 



Stone Canal. — The calcifications in the wall of the madreporic canal 

 were observed in a larval stage before the external modifications of the 

 plate through which it opens were evident or had appeared. It con- 

 sists of a delicate tubular network of calcifications, formed by a lacework 

 of calcareous spicules, which appear to arise from many centres of for- 

 mation. They appear to form in the wall of the tube itself. I am un- 

 aware that any one has described the stone canal in a young starfish in 

 which there were but seven median dorsal plates, yet it is well marked 

 at that age. 



4. Comparison with other Asteroidea. 



It is here intended to consider certain relations between the plates 

 of Asterias and the observations and comparisons which have been 

 made by others on the plates of young starfishes. The study of the cal- 

 careous formations of the adult and their history from the time when they 

 first appear has engaged the attention of several naturalists, and many 

 different conclusions have been arrived at in this stud}\ With these 

 recorded observations and interpretations I have been able to compare 

 my own on Asterias, and their concordance has strengthened my belief 



