144 McMURRICH. [Vol. V. 



the cavity. The larvae here described give important evidence 

 as to the origin of the canals of the column-wall mesogloea. 

 The mesogloea is fairly well developed in the younger speci- 

 mens, and contains a number of the canals as well as a few 

 elongated, fibril-like cells. The canals are filled with a coarsely 

 granular mass, which does not stain well, and in which no cell 

 outlines can be distinguished. It resembles very closely in 

 appearance the granular layer of the ectoderm, which has been 

 described as lying immediately exterior to the mesogloea, and is 

 in fact derived from that layer, portions of which can be seen in 

 various stages of enclosure by the mesogloea. The endoderm 

 in these young stages, on the other hand, takes no part in the 

 formation of the canals, though my preparations of the adult 

 Mammillifera tuberciilata show that in later stages portions of 

 the endoderm may be enclosed in the mesogloea, a process which 

 we know occurs in the formation of the mesogloeal sphincters. 

 As regards the basal canals of the mesenteries, my preparations 

 do not permit of any certain conclusions, though appearances 

 tend to show that their contents are also ectodermal in origin. 

 In sections passing through the stomatodaeum, the basal canal 

 is comparatively small, producing only a slight enlargement of 

 the base of the mesentery, or it may even be entirely absent. 

 Below that level, however, it is very large, giving the imperfect 

 mesenteries a pyriform shape in section (Fig. 8). The canal is 

 occupied by a granular mass exactly resembling that found in 

 the canals in the column wall, and it seems probable that it has 

 been formed in exactly the same way. In the imperfect mesen- 

 teries the canal is usually situated at the distal end of the mes- 

 entery, but in the perfect ones it is found almost equidistant 

 between the base and the free edge. In all cases a canal of 

 varying size, but usually intermediate in size between those 

 of the column wall and that of the mesentery, is to be found 

 immediately at the base of the mesentery, and in many cases 

 small masses of granular substance, evidently part of the canal 

 contents, are to be found lying outside the canal amongst the 

 endoderm cells. This might seem to indicate an endodermal 

 origin for the mesenterial canal contents, but another explana- 

 tion seems to me preferable, viz. that these masses are portions 

 of the canal contents which have escaped to the exterior, since 

 I find mesenteries in which the boundary between the canal and 



