EXPLANATION OF THE PLATES. 127 



Plates III. -VIIL Embryology of Asteracaxtiiiox tallidus A//. 



PLATE III. 



Owing to the transparency of these larvs, it is not easy to ascertain whether they are seen with the month 

 downwards or upwards, unless we ascertain the position of the madreporic IxKly. In all these fig- 

 ures, whenever the water-tube w' is on the left of the figure, the mouth is turned upwards. 



Fig. 1. The youngest larva of this species, seen from the mouth side, corresponding to PI. 11. Fig. 20; 

 a comparison of these two figures will show the great difference between the larvae of these two 

 species of Starfishes. In the former, the chords of vibratile cilia appear much earlier, and the oral 

 plastron is well defined; while, in the other species, it is not before it has reached the condition of 

 PI. II. Fig. 26, that the oral plastron is as well developed. 



Figs 2-10. Brachina stage. 



Fig. 2. A larva seen from the left profile, corresponds to the stage of PI. U. Fig. 27, of A. berjlinus; 

 with the exception of the size of the water-tubes, the larva of this species is much stouter, shorter, 

 and the anal portion is the most prominent, while the larva of the A. berylinus is quite slender and 

 elongated. 



Fig. 3. The same larva as Fig. 2. seen from the dorsal si<le. 



Fig. 4. A more advanced larva, seen from the dorsal side ; the undulations of the ciliary chord indicate 

 the future arms, the water-tubes extend beyond the mouth, and have already begun to bend towards 

 each other. 



Fig. 5. The same larva seen from the left profile, to show the bent attitude frequently assumed by the 

 larva when disturbed. 



Fig. 6, This larva, seen from the mouth side, is more developed than any raised by artificial fecundation 

 from the eggs of A. berylinus. The water-tubes have greatly increased in diameter; they have united 

 beyond the mouth, and extend on each side of the stomach so as almost to meet, but without uniting. 

 The mere indentations of the previously figured larvaj correspond to accumulations of pigment cells, 

 and to the thickening of the vibratile chord, accompanied by the formation of rudimentary lobes, 

 which indicate plainly the position of the median arms (e'), the dorsal anal (e"), the ventral anal (e'"), 

 and dorsal oral arms (e""). The greatest accumulation of pigment cells, and the thickening of the 

 vibratile chord, is found at the rudimentary median arms (e'). The anal ventral pair of arras (e'") is 

 especially well marked. 



Fig. 7. The preceding figure seen in profile, the mouth to the right, shows the great development which 

 the oral position of the water-tube has taken ; also the mode of formation of the oral ventral pair of 

 arms (e*), as well as the first sign of the odd brachiolar appendage (/"). 



Fig. 8. Larva seen from the dorsal side. The arms have increased greatly in size since the stage repre- 

 sented in Fig. 6. The oral portion of the water-tube has become very pointed ; it extends into the 

 odd oral arm (e'), which has also elongated, and stands out prominently beyond the oral plastron. 



Fig. 9. The same figure seen in profile, with the mouth downward. The vibratile chord is a deeply 

 undulating line, following the edge of the arms, which extend beyond the general outline. The 

 water-tube, it will be seen, forks also at the oral extremity ; one branch extending into the odd 

 arm (e®), the other toward the angle made by the base of this arm and the pair of oral ventral 

 arms (e^). The great increase in size of this odd arm will be seen when compared to Fig. 7 of 

 this plate. 



Fig. 10. Larva seen from the mouth side. Thus far the arms had altered but little the character of tlie 

 outline of the larva. In this figure, however, some of them are sufficiently developed to be capable 

 of extensive motion. The median arms (e') especially are far in advance of the others. All the anal 

 arms develop so as to become more slender at first, and assume their true character sooner than the 

 oral arms, which, during the early stages, are always more heavy, and take their final shape later than 

 the anal arms. At the angle, where the oral ventral arms and the odd arm come togetlier, at the 

 base of the oral arms, slight swellings are formed (/), which are the first trace of the pair of brachi- 

 olar arms (ff), the odd brachiolar arm being only seen in tlie profile 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 p.air of arms {/). The first trace of the actinal area 

 of the future Starfish is also plainly visible (/) on the water-tube (jc), on the left of this figure. 



