LARVAL FOEMS OF ECHINODERMATA. 



489 



and the emersion of the embryo ; the division into two, four, or 

 even eight segments taking 

 place within three hours 

 after impregnation. Within 

 a few hours after its emer- 

 sion, the embryo changes 

 from the spherical into a 

 sub-pyramidal form with a 

 flattened base; and in the 

 centre of this base is a de- 

 pression, which gradually 

 deepens, so as to form a 

 mouth that communicates 

 with a cavity in the interior 

 of the body, which is sur- 

 rounded by a portion of the 

 yolk-mass that has returned 

 to the liquid granular state. 

 Subsequently a short intes- 

 tinal tube is found, with an 

 anal orifice, opening on one 

 side of the body. The pyra- 

 mid is at first triangular, but 

 it afterwards becomes quad- 

 rangular; and the angles are 

 greatly prolonged round the 

 mouth (or base), whilst the 

 apex of the pyramid is some- c 

 times much extended in the 

 opposite direction, but is 

 sometimes rounded off' into 

 a kind of dome (Fig. 244, A). 

 All parts of this curious body, 

 and especially its most pro- 

 jecting portions, are strength- 

 ened by a framework of 

 thread-like calcareous rods 

 (e). In this condition, the 

 embryo swims freely through 

 the water, being propelled 

 by the action of cilia, which 

 clothe the four angles of the pyramid and its projecting arms, 

 and which are sometimes thickly set upon two or four projecting 

 lobes (/) ; and it has received the designation of Pluteus. The 

 mouth is usually surrounded by a sort of proboscis, the angles 

 of which are prolonged into four slender processes (g, </, g, g}, 

 shorter than the four outer legs, but furnished with a similar 

 calcareous framework. 



323. The first indication of the production of the young Echi- 



Embryonic development of Echinus : A, Pluteus 

 larva at the time of the first appearance of the disk ; 

 a, mouth in the midst of the four-pronged proboscis; 

 6, stomach; c, echinoid disk ; d, d, d, d, four arms 

 of the Pluteus body; e, calcareous framework; /, 

 ciliated lobes; g, g, g, g. ciliated processes of the 

 proboscis : B, disk with the first indication of the 

 cirrhi : c. disk, with the origin of the spines be- 

 tween the cirrhi : D, more advanced disk, with the 

 cirrhi and spines projecting considerably from the 

 surface. (N.B. In Figs. B, c, and D, the pluteus is 

 not represented, its parts having undergone no 

 change, save in becoming relatively smaller ) 



