(6) 



EXPLANATION OF THE PLATES. 



view, Fig. 12, /', though it can be traced as a 

 double outline of the odd arm (/")• We can already 

 see a constriction in the water-tube as it passes into 

 the odd arm, and from this (nearer the mouth) are 

 sent off two small pouches (/' /'), which enter into 

 the brachiolar pair of arms (/). The first trace of 

 the actinal area of the future Starfish is also plainly 

 visible (f) on the water-tube (w'), on the left of this 

 figure. 



Fig. 11. A more advanced larva than Fig. 10, seen 

 from the mouth side, in which the oral arms have as- 

 sumed all the characters of the anal appendages. 

 The brachiolar arms are quite well developed ; the 

 intestine and the stomach are slightly crowded to one 

 side by the greater increase of the actinal area (t) 

 of the Starfish ; the ambulacral pentagon of the 

 future Starfish is still more marked (<) than in pre- 

 vious stages. Brachiolaria stage. 



Fig. 12. The same as Fig. 10 seen in profile, with the 

 mouth downwards. 



PLATE IV. 



Fig. 1. Seen from the mouth side. A larva with its 

 arms fully developed and in full activity; no further 

 changes take place in the general aspect of the larva, 

 with the exception of those of the anal part where 

 the Starfish is developing, and those of the brach- 

 iolar arms. All the arms are nearly equally advanced, 

 with the exception of the median arms (e'), which 

 still retain their greater size. The odd terminal arm 

 (fi 6 ) has also greatly increased in length, as well as 

 the brachiolar arms (//), which are capable of mo- 

 tion, and into which the branches of the water-tubes 

 can easily be traced. Brachiolaria stage. 



Fig. 2. The same larva, seen from above, on a some- 

 what smaller scale, shows in what way the stomach 

 and the intestine have been pushed to one side, by 

 the great development of the actinal part of the Star- 

 fish, on the right of the figure ($f). The shape of the 

 mouth (m) is particularly well seen in a dorsal view, 

 at this stage of growth. 



Fig. 3. The same larva on a different scale, seen end- 

 ways, from the oral end, to show the connection be- 

 tween the pair of brachiolar arms (//) and the 

 oral ventral pair (e 5 ), as well as the position of the 

 odd brachiolar arm (/") at the base of the odd 

 terminal arm (e 6 ). 



Fig. 4. An adult larva seen from the right, actinal 



profile; the arms are in the position which they take 

 when moving rapidly, arched towards the median 

 arms, the brachiolar arms alone being curved in the 

 opposite direction from the others. Here the crescent- 

 shaped ambulacral pentagon, as well as the lobed pen- 

 tagonal outline of the abactinal area are plainly seen. 



Fig. 5. A magnified profile view of the brachiolar arms. 



Fig. 6. The brachiolar arms seen from the ventral side 

 of the larva, to show the position of the single disk 

 and of the double row of disks at the base and on 

 each side of the odd brachiolar arm, somewhat less 

 magnified than Fig. 5. 



Fig. 7. The anal part of the larva soon after the 

 shrinking of the arms has begun. The whole of the 

 terminal anal part of the" larva has gradually been 

 absorbed, so that the disk of the Starfish occupies 

 the whole of the space between the median arms, 

 seen from the ventral side ; the oral extremity of the 

 Brachiolaria is unchanged and not represented. 



Fig. 8. The shrinking has gone so far that the whole 

 of the anal part has been affected, and the oral ex- 

 tremity alone, with the brachiolar and the terminal 

 arms, retain their original shape and proportions. 



Fig. 9. A different view of the anal part of a larva from 

 that of Fig. 7 ; in a slightly more advanced condition 

 than that of the preceding figure, showing the great 

 height of the abactinal region of the young Starfish; 

 the oral extremity of the Brachiolaria is omitted, as it 

 remains almost unchanged. 



Development of the Starfish proper. 



The Figures on this Plate show the gradual development 

 of the actinal and abactinal regions of the Starfish, 

 and the figures represent simply the anal part of the 

 Brachiolaria, which is alone affected during this devel- 

 opment. 



Figs. 1-7 correspond to a Brachiolaria, having reached a 

 'state about as advanced as that of PI. LIT. Fig. 10. 



Fig. 8 is a Starfish developed on the Brachiolaria of PL 

 IV. Fig. 11; while Figs. 9-14 are stages of develop- 

 ment which are only found on Brachiolarise having 

 their full complement of arms, and in which, except 

 these changes of the Starfish, but slight modifications 

 take place. 



Figs. 1, 2, 10, 12 represent that profile of the anal part 

 of the Brachiolaria, in successively more advanced 



