ECHINODEEM LARV^. 7 



The eggs, either before or after fertilization, are shed into the water and are attached 

 to the mother's back, possibly by means of a mucoid secretion. They are soon, 

 however, surrounded by the brood-pouches, which probably develop by a sinking-in of 

 the dorsal ectoderm, and here they are retained until they have reached a condition 

 where the tentacles are able to serve as organs of attachment to the mother. This we 

 know must be when the little animals are between 3 mm. and 5 mm. in length. They 

 then pass out of the brood-pouches and are arranged along the dorsal ambulacra of the 

 mother as Thomson has described. 



The presence in the sacs, among a large number of older embryos, of two which 

 are comparatively young, naturally brings up the question of the number of broods in 

 this species. Are all of the young brought forth in a single brood, or is there a 

 succession of broods such as we find in Synapta vivipara, Clark (3) ? If these two 

 embryos represented the beginning of a second and younger brood, one would expect 

 to find the gonad of the mother showing some sign of activity ; but, as this is not the 

 case, we must conclude that they are members of the same brood as the larger embryos, 

 and that their development has been, for some reason or other, retarded. What 

 evidence we have, therefore, seems to be in accord with Thomson's observation that all 

 the young attached to one mother were of the same age and size. 



Coming now to the embryos themselves, it is somewhat disappointing to find that 

 among the large number brought back only two stages are represented. Turning our 

 attention to the younger of these, we see that of the two specimens in the collection 

 one was somewhat distorted through the pressure of the surrounding embryos in the 

 brood-pouch ; but this does not at all affect the disposition of the internal organs as 

 seen in sections. The other specimen, apparently quite normal, is oval in form, and 

 measured before sectioning 1 • 8 by 1*2 mm. No evidence of any external opening 

 could be made out. 



When sections of this stage are examined it is seen to be a typical pentactula, and 

 the only points which need be specially mentioned are in connection with the water- 

 vascular system. The ring canal is well developed, and the radial canals coming off 

 from it can easily be followed as they arch back and run towards the aboral pole. As 

 yet there is no sign of the development of podia, but the tentacular canals can be seen 

 to originate one on either side of each radial canal, close to where it comes off" from the 

 ring canal, and to run out into the tentacles which do not reach the surface of the 

 embryo, but still project into the " atrium." Even at this stage valves may be seen 

 guarding the entrance to these tentacular canals. The stone-canal is well formed, and 

 runs out in the dorsal inter-radius to open into a large, clearly-defined axial sinus. 

 From the latter the pore-canal can be traced outwards for a short distance, but we were 

 not able to follow it to its external opening in this stage, though in the older embryos 

 it is very easily followed. No Polian vesicle has yet been developed. 



Of the later stage there were a great number of specimens, for it was found that 



all the embryos in the brood-pouches (with the exception of the two already mentioned), 



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