Chap. III. HYDRACTINIA POLYCLINA. 237 



geneous layer. In the older portions of this layer these cells are very irregularly 

 arranged {Figs. 5% d, and 5", b), and appear like imbedded crystalline bodies. In 

 a young hydroid with two tentacles [Fig. 5), the cells {a}) of the outer wall are 

 already very faint. The outer wall of the adult hydroid is so excessively trans- 

 parent, that we have not been able to discover any thing more than a faint 

 indication of large, broad cells ; these are most satisfactorily seen in the proboscis 

 {Fig. 2'', a). The outer wall of the tentacles {Fig. 2\ a) of the fertile hydroids, 

 either male or female, is so thickly beset with lasso-cells that they appear to be 

 the sole component of the layer in which they are imbedded. Lasso-cells, identical 

 with these, are scattered all over the outer Avails of the hydroids and the me- 

 dusoids {Figs. 3, 7, l. and 8, I). The intimate structure of the long, cylindrical 

 tentacles of the sterile form, has not been carefully investigated, but enough has 

 been seen to warrant us in saying that they very closely resemble those of Clava. 

 The inner wall {Fig. S"", l^) of the young hydroid has a close resemblance to the 

 outer one («'), but the cells are more regular and columnar. They, too, form but 

 a single columnar layer, the inner ends abutting on the chymiferous channel (c). 

 In the anastomosing chymiferous canals {Fig. 5*, e; PI. XXVI. Fig. 18, a) these cells 

 are almost as irregular, both in shape and arrangement, as in the outer wall ; 

 still, their longer diameters have a greater trend toward the centre of the canal 

 (PI. XVI. Fig. 5^ d, wood-cut 34, a, p. 231, and PI. XXVI. Fig. 18, a, Vol. IV.). 

 The inner wall (PI. XVI. Fig. 3, b) of the adult hydroids has a columnar structure, 

 like that represented in the proboscis {Fig. 2'', b), consisting, through the whole 

 length of the body, of broad columnar cells, each one of which extends from the 

 outer to the inner surface of the wall. 



The Lasso-cells. — There are two kinds of lasso-cells imbedded in the outer walls 

 of the hydroids and medusoids, one of which is much larger than the other. The 

 larger ones {Fig. 11, a, b) are very small, when compared with those of the Polyps, 

 and when seen with a magnifying power of six hundred diameters, appear to the 

 eye to be about one seventh of an inch long. They have an oblong-oval shape, 

 slightly narrowed at the open end {Fig. 11, a). Professor Clark has ascertained 

 that the interior contains a spiral coil of filament and a central thick column, 

 which bears the same relation to the spiral coil as obtains in the lasso-cells of 

 Cyanea, Aurelia, Coryne, and the Polyps generally.^ When the lasso is extruded 

 {Fig. 11, b, b\ c) we see that it differs from that of any other hydroid, and has the 

 character of that of the strobiloid Medusae and of Polyps. The everted central 

 column {Fig. 11, b^) is elongate fusiform, and has about the same length as the 

 cell from which it is protruded. It is endowed with a double spiral row of cilia ; 



' See the remarks of Prof. Clark on this subject, and especially on the lasso-cells of Coryne, p. 20D. 



