OCR Text |
Show 60 PROF. F. J. BELL ON THE [Feb. 19, bear them are all distinct, the spines along the middle line of the arms are beginning to grow up, hut only one interradial spine is as yet developed ; the spines on the ventral surface are now more regularly arranged. In the third or largest specimen (y) all the spines-those on the distal marginal plates, those on the middle line of the arm, the central portion of the disk, and those that lie between the interradial lines-are all long, strong, sharp, and prominent. It is clear, therefore, that we have here to do with a condition in which the spines increase in size and number during the growth of their possessor; this is to be insisted on, inasmuch as it is not a condition which always obtains. In some species of Linckia we find that the spines diminish in size as the form grows larger; hut. in that case we also see that increase in size is accompanied by consolidation of the skeletal plates-or, where spines are wanting and plates are not very strong, arms are often found to have been broken off or injured. The study of individual development and the consideration that the larger the form the greater its need of defence, lead to the supposition that the least modified Oreaster will be found to be one that is not specially spinose. This consideration gains in force when we know that a form with feebly developed spines such as O. nodulosus has in the most striking fashion the characters of the younger repeated in the older individuals. In the arrangement, therefore, of the species of the genus, we shall have to commence with those which have the spines least well developed. Next to the non-spinous condition of such a form as O. nodulosus, we should probably place those in which a few spines are developed at the proximal end of the arm to defend, so far as may be, the more central, and thereby more precious, portion of the internal organs. The next line of defence is probably that of the free end of the arms, next the ventral plates, and lastly the dorsal surface. On the other hand, we have in Oreaster occidentalis an example of a species in which the marginal spines disappear during growth. Not only have we evidence of this in Prof. Verrill's original description of the species, but more convincing proofs are afforded by the three specimens in the collection of the British Museum : the smallest of these, one presented and named by Mr. Verrill, never has more than five of the supermarginal plates on either side of any arm without spines ; and a few are also to be found on the infero-marginal spines. The smaller specimen described by Verrill has "from one to four (upper plates) that bear small, short, stout, bluntconical spines near the end of the rays; " this is very much the condition in which I find a specimen collected by Mr. Lock-ington in San Francisco Bay. The larger specimen described by Verrill had no spines on the superomargiual plates ; while a yet larger specimen collected by Lockington has on the terminal plate of some of the rays three minute processes, distinctly smaller than those of the other specimen from the same collector. |