CHESTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 767 



axis (a single case) did not by its upward growth push before it the 

 floor of the digestive cavity of the primary polyp, but rather grew 

 upward in the wall of the cohnnn at one side, so that tiie primary 

 polyp had tiie appearance of being a lateral outgrowth from the axis. 

 From this he concludes that the stem of the colony in Anthoplexaura 

 apparently does not belong to the primary polyp, but to the coenosarc 

 (at its base), just as in Pseudaxonia. 



The existence of a secreting epithelium in the adult of Pseudo- 

 plexaura cannot be doubted. The axis-secreting cells are large, and 

 this cell layer, which is evident, can be traced in free-hand sections 

 from the tip of the axis to the spreading base near the substratum. 

 I always find an unbroken axis epithelium around the tip. This 

 seems to me to be irreconcilable with the method of growth outlined 

 by Studer, and represented by the sections shown in A. Schneider's 

 paper. For Studer's theory demands a mass of spicules at the tip, 

 as well as in other places, perhaps, — spicules which are later to be 

 resorbed. These spicules must of course develop in the mesogloea, 

 and for their incorporation into the axis would require a break in the 

 epithelium around the tip; but such a break I have not seen in Pseu- 

 doplexaura. There is often a massing of spicules at the tip outside 

 of this epithelium, but there is no trace of their inclusion in the axis, 

 nor of their conversion into it. The spicules are here pushed aside 

 by the growth of the axis and remain as spicules in the mesogloea. 

 A. Schneider holds that the axis epithelium as figured by von Koch, 

 with which the epithelium of my figures is undoubtedly homologous, 

 is the endodermal lining of the digestive cavity of the axial polyp, 

 into which the axis has been pushed, and that the longitudinal canals 

 are mesenterial chambers. But so far as regards Pseudoplexaura, 

 the cells of the axis epithelium are not like endoderm cells. Moreover, 

 the longitudinal canals vary considerably in numbers in Pseudoplex- 

 aura tips, being eight or more; besides, as Kinoshita found for Antho- 

 plexaura, they have no muscles and sometimes end in solenia. 



Pseudoplexaura, then, affords no evidence of spicules included in 

 the axial skeleton; a secreting axis epithelium is present, the cells of 

 which are unlike those of the endoderm in their arrangement and 

 structure. Phen when pushed aside by the spreading of the desmo- 

 cytes, they are not easily to be mistaken for endoderm cells. Kino- 

 shita's evidence in the embryos of Anthoplexaura is a strong support 

 for the ectoderm theorv. The results from the studv of the adult of 

 Pseudoplexaura are not in themselves complete evidence, but so far as 

 they go speak strongly for the ectodermal origin of the horny axis, 

 as indicated by von Koch. 



