No. 3.] MORPHOLOGY OF THE ACTINOZOA. 317 



of glandular elements, which characterizes the adult (McMur- 

 rich, '89 a). At the aboral pole there is a peculiar differentiation 

 of the ectoderm which is not found elsewhere on the body. 

 Immediately external to the mesogloea, which is well developed 

 at this stage, there is to be seen in longitudinal sections of the 

 embryo, a layer (Fig. 13, ;/) much more faintly stained with 

 haematoxylin than either the mesogloea {mg) or the portion of 

 ectoderm {ec) which lies superficial to it ; it is crossed by numer- 

 ous fine fibrils, in the meshes of which lies a clear, faintly gran- 

 ular substance, which does not stain at all, or but very faintly. 

 The fibres are readily seen to be delicate prolongations of the 

 mesogloea extending up into the ectoderm, and appearing in 

 cross-section as deeply stained, round, homogeneous dots, and 

 the matrix in which they lie is apparently composed of nerve- 

 fibrils. It occupies the relative position held by the nerve-layer 

 in adults, and it is probable that it is of that nature, although I 

 have not been able to distinguish ganglion cells in it. In swim- 

 ming, the aboral end is anterior, and the special development 

 of sensory cells and a nerve-plexus at that pole is not surprising. 

 The layer is distinguishable only for a short distance up the 

 sides of the body, though probably it extends in a very much 

 less developed condition all over the body. The special devel- 

 opment at the aboral end must be considered a larval adapta- 

 tion. 



The endoderm consists of large, vacuolated cells, of the same 

 nature as those found in the adult. They contain large num- 

 bers of Zooxanthellae, which occur, however, also in the ecto- 

 derm, and the large nematocysts, which I described as occurring 

 in the endoderm of the adults, are present in the embryos only 

 in the ectoderm. The endoderm cells in the younger stages 

 completely fill up the central cavity, and show little or no 

 arrangement into a definite layer. It seems probable from this 

 that the formation of the germ-layers in this form differs some- 

 what from that described for Metridium, and more nearly ap- 

 proaches what occurs in the Alcyonaria and in Manichia. 



In the youngest specimen obtained, the first pair of mesen- 

 teries is in process of formation. The number of embryos at 

 my disposal is not sufficient to trace out fully all the processes 

 concerned in the formation of these mesenteries, but I can 

 merely deduce what is presumably taking place from conditions 



