MUSEUM OF COMPARATIVE ZOOLOGY. 145 
stand it, but to special plates not represented in the starfishes, Asteracan- 
thion, or Asterina, young or old. 
Dorsal Plates. — The dorsal or upper plates of the arm (Fig. 19, d) 
originate in the median dorsal line as simple trifid spicules, and form in 
series from the adoral to the terminal, the adoral being the oldest. The 
last formed is nearest the terminal. In their first condition they resem- 
ble the ventrals, and the subsequent growth is similar. The dorsal ap- 
pears after the corresponding ventral. This fact is an interesting one in 
comparative anatomy. Many genera of Ophiuride (Ophiohelus, Ophi- 
ambyx, and others from deep seas) and all the Astrophytide are destitute 
of dorsals. In these Ophiuride the ventrals are present, and in the 
Astrophytide the first ventral is developed while others are wanting. 
The dorsals disappear before the ventrals, if the want of dorsals in these 
low genera is due to degradation, or the genera have not progressed 
through embryonic stages in which dorsals appear, if, as is probably 
the case, dorsals have never appeared. In the growth of Amphiura the 
ventrals form first, and those genera with a single ventral and no dorsal 
may be compared with my Fig. 17, Pl. IIT. 
I am led to suppose that the dorsals have been inadvertently omitted 
in certain of the figures of a young Amphiura by Ludwig (Pl. XI. Figs. 
21, 25), for he has not represented these plates in a young specimen in 
which three pairs of side arm-plates are represented (Pl. XI. Fig. 21, ad’, 
ad‘, ad*). Ina young Amphiura of about the same age (Pl. III. Fig. 19) 
at least one dorsal plate is formed, and in another as old as that repre- 
sented in his Fig. 25 (same plate) the dorsals have increased in number. 
In none of Ludwig’s figures are dorsals represented, although in Figs. 21, 
25, they must have been already formed. 
_ The dorsal originates adaxially to its side plates on the median line, as 
shown in my figures. 
would explain the position of the water-vessel in the former group. We must look 
with interest to the method of growth of these plates in sea-urchins much younger 
than any yet studied for a solution of this question. 
As the viviparous Amphiura and the Asterina with the direct development have 
thus far furnished the best information in regard to the method of formation of the 
plates in Ophiurans and starfishes, perhaps the growth of the plates in Echinoids 
can best be revealed by the life history of the Hemiaster of the South Sea, with 
young in pouches in the ambulacral zones. Although the young Hemiaster has 
been well described by A. Agassiz, Thomson, and others, the growth of the young 
plates is as yet not well enough known for comparison. 
VOL, XIIIL—wNO. 4. 10 
