124 BULLETIN OF THE 



is here regarded as the dorsocentral, is developed after the oculars or 

 genitals,* and that the dorsocentral is formed axially to the same. In 

 Toxopneustes, according to authority mentioned, the facts seem to be 

 diametrically opposite those mentioned for Echinarachnius, while the 

 last-mentioned genus resembles Amphiura so far as the time of the devel- 

 opment of the dorsocentral is concerned. 



The dorsocentral of the starfish, Asteracanthion, is apparently formed 

 in some cases after the radials (oculars) and genitals. In Asterina the 

 dorsocentral seems to coexist with the other ten plates from the first, as 

 shown in Lud wig's figures, although he does not say whether the dorso- 

 central appears before or after the other ten. Loven's figure of the young 

 A. glacialis (op. cit.) is not young enough to answer our question. In 

 A. Agassiz's Embryology of Asteracanthion it is shown that the ten cal- 

 careous rods are the first to form, and attain a considerable size before the 

 dorsocentral is represented. Metschnikoff gives very instructive stages + 

 of the young of an unknown Asterid ; but his description is so short, and 

 the arrangement of plates in it so remarkable, that it would be out of 

 place to interpret them here. This stage and those preceding it give no 

 answer to the question when the dorsocentral is formed. The question 

 which plates in the young Amphiura correspond to the oculars of sea- 

 urchins assumes a new phase in the light of what we know of the perma- 

 nent retention of the radials in the abactinal hemisome of the body of Am- 

 phiura, and the relation of the terminals to the primitive tentacle. Now 

 that we know that the primary radials of Amphiura are not pushed out to 

 the extremity of the rays, but always remain in the disk, and that another 



* Consult PI. Vn. Fig. 16, Bull. Mus. Comp. Zool., Vol. XII. No. 4. This 

 figure is thought to show the truth of the belief stated. No limestone rods were 

 observed in C2)l, which occupies the position of the suranal or dorsocentraL 



+ The figures alluded to are found on PI. XII. Figs. 1 and 2 (Studien liber die 

 Entwickelung der Echinodernien und Nemertinen). Fig. 1 is an abactinal view, 

 with five large peripheral, spiniferous plates, and a madreporic plate (JIfy). Within 

 this ring of jseripheral plates are seven smaller, non-spiniferous plates, one of which 

 is centrally placed. Metschnikoff was unable to determine the genus to which this 

 starfish belongs. He found the absence of the anus to recall Astropecten, Luidia, or 

 Ctenodiscus, but supposes that the character of the spines does not belong to these 

 genera. With this objection, so far as the size and general arrangement of these 

 huge spines are concerned, I cannot agree with him. I have studied the young of 

 our C. crispatus, and find the five peripheral plates which he figures, each with 

 three large conical spines, a median and two lateral, on each plate. Between each 

 pair of these peripheral plates, which are regarded as the terminals, there are three 

 adambulacial plates in each interradius, while in the stage figured by Metschnikoff 

 there is but the single madreporite in this position. 



