THE STRUCTURE OF XENTA HICKSONI. 257 



Ectoderm. 



The ectoderm is a moderately thick layer, iu which large 

 columnar and smaller interstitial cells may with difficulty be 

 distinguished. If a section of a tentacle or pinnule, which 

 was well expanded at the moment of killing, be examined, the 

 ectoderm is then seen to consist of a row of cells elongated at 

 right angles to the free surface, below which are smaller 

 rounded cells which probably correspond to the interstitial 

 cells of the ectoderm of Alcyonium and other coelentera. 

 (PL 25, fig. 11). 



The protoplasm of the ectoderm cells is very finely granular^ 

 occasionally cells are met with containing a few small vacuoles. 

 Many of the ectoderm cells of the tentacles and pinnules are 

 produced at their inner ends into muscle processes which lie 

 in the outer portion of the mesogloea, parallel to the free 

 surface. These muscles are all longitudinal in direction. The 

 ectodermic muscles of the tentacles are much more strongly 

 developed on the oral than on the aboral side, especially at the 

 base of the tentacles, where the muscles of the oral side form 

 a strong band beneath the cells quite eight times as thick 

 as the band of muscles on the aboral side. Nearer the tip 

 of the tentacle the muscles of the two sides become almost 

 equal. 



In the body of the polyp ectodermic muscles are present 

 only in the distal portion arouud the base of attachment of 

 the tentacles and for a distance of about a millimetre below 

 this point. Myo-epithelial cells are absent from the ectoderm 

 of the stem. The absence of muscle- cells from the ectoderm 

 of the stem and the greater portion of the body of the polyp is 

 connected with the non-retractile character of these parts. 

 Their presence in the tentacles, pinnules, and distal portion of 

 the body of the polyp confers on these parts some power of 

 contraction, and an examination shows that the pinnules and 

 tentacles vary somewhat in length and shape, and that the 

 tentacles are often turned inwards over the mouth, due to the 

 contraction of the muscles of the oral side. The absence of 



