MUSEUM OF COMPARA.TIVE 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 Ophiuridse (Ophiohelus, Ophi- 

 ambyx, and others from deep seas) and all the Astrophytidae are destitute 

 of dorsals. In these Ophiuridae the ventrals are present, and in the 

 Astrophytidae 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, PI. III. 



I am led to suppose that the dorsals have been inadvertently omitted 

 in certain of the figures of a young Amphiura by Ludwig (PI. XL Figs. 

 21, 25), for he has not represented these plates in a young specimen in 

 which three pairs of side arm-plates are represented (PI. XI. Fig. 21, ad^, 

 ad*, ad^). In a young Amphiura of about the same age (PI. 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 Lud wig'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 ambulacra! 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. XIII. — NO. 4. 10 



