320 HYDROID/E. Part IV. 



opposite the walls (jf the axial cells {Figs. 20 and 21, 1?), which are arranged 

 end to end in a single series, and none are opposite the cells themselves, so that, 

 as a natural conse(|nence, they are not only arranged in longitudinal, but also in 

 transverse rows or circlets. The proboscidal actinostome exhibits very clearly the 

 changes which the component cells undergo during its contraction and expansion. 

 During contraction they are more nearly equilaterally polygonal {Fi<j. l-t), and do 

 not seem to have any method in their arrangement; l)ut in expansion {Fig. 12, />) 

 they are disposed in rows, radiating from the centre, and are elongated {Fig. 14) 

 in the same direction. Even with a magnifying power of five hundred diameters 

 they appear small, but yet very well defined in outline. The cells of the lower, 

 or actinal, surface of the disk {Fig. 15) are a little larger than the last, and 

 differ in having finely granular contents. In the double, bulb-like protrusions 

 {Figs. 21, r, and 21") of the edge of the disk, at the base of each tentacle, 

 the cells are much smaller than those of the actinostome, but, nevertheless, sharply 

 polygonal. 



Proles hj/droidca. — Whenever a new branch, or a new pedicel begins to Ijud, 

 the cells of the outer and inner walls of the old branch become quite con- 

 spicuous at that point (PI. XXXIII. Fig. o'\ rt\ I?), and to some distance above 

 and below it (t), whereas, on the opposite side, they are not more prominent 

 (a, (i) than usual. In a later stage {Figs. 3 and 4, 4"), we will describe the 

 peculiarity of these cells. It will be noticed that the wall of the bud {Fig. 3", 

 a}, i') and of the old stem near it (<■), are considerably thickened, and press closely 

 against the chitinous sheath ; and that the latter is torn open and cast aside (c^) 

 by the protruding bud, which bears a new sheath (e'') of much thinner and more 

 delicate structure. From the beginning, the bud has a tendency in the direction 

 of its future line of groAvth, and even overlaps (cV) the main stem (*) to a 

 considerable extent. As the stem or peduncle grows, it assumes at each point 

 the form which it ever after retains, as may be seen by the examination of our 

 figures. Fig. 3 represents a young branch which, at the lower part, has all its 

 adult characteristics, as regards the general proportions (a, (-i, y, e); while at the 

 end it is still growing, and, as it proceeds in this way, the rings are developed 

 by a deposition from the exterior surface of the outer wall («'). As fast as the 

 chitinous tube is completed, the outer wall withdraws from it, in a greater or less 

 degree, leaving here and there isolated projections (j) still adhering to the sheath. 

 The completion of this tube corresponds, also, to the obscuration of the cellular 

 structure of the outer and inner walls, in fact, the amount of development of the 

 stem may be estimated by the degree of faintness of the cells of these walls. 

 The chymiferous cavity does not follow the terminal growth of the stem very 

 closely, as the tip of the inner wall of the young part of the pedicel is solid 



