120 BULLETIN OF THE 



body where the umbilicus is joined to the edge of the disk (Fig. 12). 

 Metschnikoff has figured them in the same position in Fig. 17 of his 

 Plate IV. Tliere seems abundant evidence that they are absorbed into the 

 body of the forming Ophiuran, as in Ophiothrix and Ophiopholis. 



III. THE PENTAGONAL EMBEYO. 



There are two forms or conditions in which the pentagonal embryo is 

 found. The one (Fig. 12) of these follows in age close upon the bilateral 

 larva, and is still attached to the mother ; and the latter is represented 

 by those larval stages when the young Amphiura has severed its connec- 

 tion with the parent and is leading an independent life (Fig. 13). The 

 exact time at which the umbilicus is broken was not observed, but the 

 rupture takes place certainly a relatively long time before the young 

 leaves the parent. The young larva then seems for some time to live free 

 in the parent without umbilical connection. 



It becomes an interesting physiological question to determine the mode 

 of alimentation of a young Amphiura in the body of the parent, and with 

 no real connection with the adult. It would seem as if in older stages at 

 least the young might be nourished by foreign matter taken into the open 

 mouth. The genital slits of the parent would furnish an easy entrance 

 for foreign bodies or small animals into the sacs, and the young Aniphiurae 

 while free in the body of the parents probably use these bodies as food. 



A. THE DISK. 

 1. Plates of the Abactinal Eegion of the Disk. 



Dorsocentral and Radials. — The aboral integument of the disk has 

 embedded in it in young stages of growth six reticulated calcareous 

 plates. These plates consist of a single central, dorsocentral {dc), and 

 five radials, the primary radials or radialia {rp). These plates have been 

 observed, figured, or described by Krohn,* Schultze,t Lyman,:]: A. Agas- 



* Op. cit., p. 34L Krohn figures (PL XIV. Fig. 1) a cross-shaped dorsocentral 

 surrounded by a continuous calcareous ring with no separate radials. This " einzigen 

 Stiicke bestehende Kalknetz " gives rise to " fiinf Schuppen." The radials, accord- 

 ing to a note on this page, are isolated from the beginning. 



t Op. cit., p. 41. 



::: OphiuridiE and Astrophytidse, Old and New, Bull. Mus. Comp. ZooL, Vol. III. 

 No. 10, p. 264. 



