320 MCMURRICH. [Vol. IV. 



Manicina, the filaments of the first pair not appearing until a 

 very much later stage of the development. In Manicina a down- 

 growth of ectoderm pushing away the endoderm occurs while the 

 stomatodaeum is in contact with the column wall ; in other words, 

 the filament makes its appearance before the mesentery. In 

 Rhodactis this is not the case, but the mesenteries are well 

 developed, and the second pair well formed and even perfect, 

 before the formation of the filaments begins. They appear to 

 be down-growths of the stomatodseal ectoderm, extending down 

 some distance below the lower end of the stomatodaeum. It 

 must be noticed, however, that the histological characteristics 

 of the down-growths are not quite identical with those of the 

 stomatodaeum, and I cannot be certain that there is histological 

 continuity. 



In one specimen only was there any trace of the filaments of 

 the second pair. Eight mesenteries were formed, two pairs 

 only being perfect. Here I found not the slightest trace of any 

 reflection of the ectoderm, but the filament, which was devel- 

 oped only in one of the pairs, was apparently simply a slight 

 down-growth from the stomatodasal ectoderm. It developed 

 in exactly the same manner as the filaments of the first pair, 

 and there is not the slightest indication in this specimen of a 

 reflection of the stomatodaeal ectoderm. In a younger speci- 

 men, however, in which only the filaments of the first pair have 

 begun to form, there is a peculiar formation of the lower end of 

 the stomatodaeum (Fig. 17), which is somewhat different from 

 what Wilson describes. The stomatodaeum on one side is split 

 from below upwards for a short distance, and the ectoderm of 

 the edge of the slit is reflected for a short distance. The slit 

 figured is between the mesenteries of the second pair, and the 

 ectoderm is reflected so as to enclose their extremities, but is 

 not continued down at all to form mesenterial filaments. On 

 the side of the stomatodaeum, opposite the slit and between the 

 mesenteries of the first pair, is a piece of reflected ectoderm, 

 which is the upper edge of a slit similar to, but less in extent 

 than, that of the other side. The reflected ectoderm of this 

 side seems to be continuous with the mesenterial filaments of 

 the first pair of mesenteries. In estimating the significance of 

 these ectodermal reflections it must be remembered that even 

 in adult individuals of various forms reflections of the lower 



