264 HYDROID^. Part IV. 



columnar structure {Fiff. 19% b). When the peduncle is extended {Fie/. 19) the 

 cells {b) of the inner wall are about as broad as long, with rounded outer, and 

 flattened inner ends. They are, like those of the outer wall, very transparent. 

 When viewed endwise, they show their polygonal shape {Fig. 18% b), and nearly 

 equal-sided outline. As the peduncle contracts, these cells become flattened 

 laterally and in the direction of the length of the wall, so that their polygonal 

 outline elongates traversely {Fig. 18, b) to this. 



The goimg Hydroid. — When the germ-mass is nearly ready to begin the process 

 of segmentation, its cellular structure is very distinct. The whole mass {Fig. 9, e, 

 Fig. 9% b) is composed of small vesicles {Fig. 9*), congregated without order. Each 

 vesicle or cell consists of homogeneous contents {a), with a mesoblast (i) which is 

 so large as to occupy three fifths of the diameter of the whole cell. This leaves 

 between the cell wall and the me.soblast so little space, in the shape of a broad 

 ring (rt), that one might very easily mistake the ring for a thick wall, and 

 the mesoblast {b) for the homogeneous contents. After a segment has separated 

 {Fig. 11, e) from the germ-basis, a greater portion of its constituent cells {Fig. 

 24, a) retain for a short time the same structure as we have just described, but 

 at the periphery of the segment the cells are very much changed ; they are 

 much smaller {Fig. 24, c) and more numerous, and densely crowded into a thick 

 layer, which in profile appears like a semi-transparent margin. As the young 

 hydroid begins to take on a polygonal shape {Fig. 21), this layer increases in 

 transparency {Fig. 21% c) and the cells {Fig. 25, e), although augmented in size, 

 become quite inconspicuous, except under the highest magnifying powers. There 

 does not as yet seem to be any arrangement among them, but, on the contrary, 

 they are packed together indiscriminately. The interior portion {Fig. 21% a) has 

 undergone a considerable change at the coi-ners («'), where the tentacles are now 

 forming. This change consists in an increase of transparency of the cells at this 

 place, and a diminution of their numbers. Soon after this, when the young hydroid 

 has assumed a prominently polygonal outline {Fig. 22), the cells of the outer 

 wall {Fig. 22% c) arrange themselves in a single layer. Each cell is about as 

 broad as long, and the outer and inner ends form, respectively, the outer and inner 

 surface of the wall. The contents of these cells are perfectly homogeneous and 

 hyaline. The inner mass, in the region of the tentacles {Fig. 22% a'), is still more 

 transparent than in the last phase, but elsewhere, throughout the body, the cells (a) 

 are still very conspicuous, so that, under a low magnifying power, they appear 

 like coarse granulations {Fig. 22, a). By the time the tentacles have grown to 

 a prominent triangular shape {Fig. 23, b), the cells of the outer wall {Fig. 23% c) 

 have grown to a full and clear definition of their respective outlines. The inner 

 mass, as far as it enters into the composition of the tentacles, has assumed an 



