192 J. H. ASHWORTH. 



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 the 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 around 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 some- 

 what 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 spicules from the tentacles and distal portion 

 of the polyp renders the ectoderm softer and more pliable, and there- 

 fore more readily acted upon by the contraction and expansion of the 

 muscle processes of its cells. 



Among the ordinary columnar cells there are in the ectoderm of the 

 stem and the body of the polyp large swollen cells which probably 

 secrete the mucus which thinly covers the external surface of most of 

 these parts (PI. XXI., Fig. 17, Muc. C). These mucus-cells are large 

 and abundant in the angle of the Y-shaped piece formed where a stem 

 divides. 



The ectoderm cells present on their outer side a moderately plane 

 surface, but on the inner side a more irregular one, as numerous 

 processes pass from the cells inwards and establish communication 

 either with the endoderm or with cells lying deeper in the mesogloea 

 (Fig- 17). 



On reaching the base of the colony the ectoderm curves inward, and 

 was applied to the face of the rock to which the colony was attached. 



