252 HYDROID^. Part IV. 



the transverse diameter is not more than half of that which is included between 

 the upper and the lower sides ; but passing toward the tip this disproportion 

 grows less and less, till the sides become nearly equal (PL XXIIP. Fig. 3). At the 

 base the sides are not so flat as toward the end, but curved in such a manner 

 that a transverse section of the tentacle would show a wedge-shaped figure, with 

 slightly convex sides, and the narrower end vippermost. At midway between the 

 base and tip a transverse section (PI. XXIIP. Fig. 3) is parallelogramic, with the 

 narrower ends alike, and corresponding to the upper and lower surfaces, the corners 

 (i*) are rounded, and the lateral longer sides (c^) are slightly hollowed. Close 

 to the end of the tentacle, just before it rounds off, the sides are more nearly 

 equal and the corners moi-e rounded than at the lower part. The sides of the 

 tip of the tentacles are compressed, so that in a transverse section the figure 

 would be broadly ovate. The end is blunt, and obliquely rounded ofl", from the 

 upper toward the lower side, so that the extreme tip is nearer the inferior surface 

 of the tentacle. 



There ai'e two layers, of different kinds of cells, in the tentacles ; the one in 

 the centre (PI. XXIIP. Figs. 1, e, and 2, e) is solid, and broader than deep (PI. XXIIP. 

 Fig. 3, e), with four straight sides, in a transverse section, and occupies two thirds 

 of the transverse diameter of the whole tentacle, and almost one half the diameter 

 at right angles to this ; the outer layer («), which forms a sheath to the inner, 

 is thinnest at the sides {^Fig. 3, c^), being about one sixth the thickness of the 

 whole tentacle, and gradually thickens above and below ; above («) it is one quarter, 

 and below {ci}) almost one third as thick as the whole mass of the two layers 

 in this direction. In consequence of this arrangement, the tentacles appear unsym- 

 metrical when seen in profile {^Fig. 1), the greater portion of their central layer («') 

 being situated above the axis. The Avhole sux'face of the tentacles is thickly studded 

 with lasso-cells, but on the under side (PI. XXIIP. Fig. 1, c^) they are much 

 more, numerous than above (c), and at the tip still more crowded. The walls 

 of the saucer-shaped disk (PI. XXIII. Fig. 1% d), when the large digestive cavity 

 which they enclose is taken into consideration, are comparatively thinner than 

 those of the stem and of the upper part {Fig. V. ^') of the proboscis. The 

 digestive cavity has a double convex shape, such as would be produced by putting 

 two watch-glasses together face to face ; above, there is a gradually narrowing 

 opening which leads to the mouth, and below, another opening which leads to a 

 suddenly expanding globular cavity {^Fig. l*", rf^) at the end of the stem {d). The 

 lower aperture of the digestive cavity corresponds to the constriction which divides 

 the base of the head from the globular end of the stem. The end of the stem 

 {^Fig. l**, d^) has a great deal of plasticity, and may assume an elongate, or very 

 broad, flattened spheroid shape; but it is, usually, nearly spherical. It embraces 



