Chap. IV. PARYPHA CROCEA. 255 



21, h c), but this want of symmetry soon disajjpears. At this time a certain portion, 

 at the exterior, becomes difTerentiated, and forms quite a thick layer {Figs. 21, c, and 

 21", c). Traces of this change may be seen earlier than this, while the separated 

 mass is in a spheroid condition {Figs. 11, e, and 24, c); the parietal portion is then, 

 to a considerable depth, more transparent than the central one ; but a marked sepa- 

 ration of an exterior thick layer does not occur till the angles appear at the edge of 

 the disk. At the angles, and just below the thick layer, the granule-like cells {Fig. 

 21", a}) are much more transparent than those along the sides, and more inwardly («). 

 Soon the sides become more equal, among each other, in length {Fig. 22), and 

 quite concave, and the angles {b), consequently, more prominent, the exterior layer 

 more transparent {Figs. 22, c, and 22", c), and the clear prominence {Fig. 22", a}) 

 of the interior still clearer. We have, thus far, sufficient evidence to show that 

 this polygonal, free body, is a young hydroid, and on this account must consider 

 the granular mass, of which it was once a- portion, as the germ-basis. The exterior 

 thick, transparent layer, is its exterior wall {Fig. 22, c), already evincing traces of 

 large, broad, columnar cells {Fig. 22", c) ; the prominent angles, lying as one plane 

 around the edge of the disk, are the tentacles {b) at the base of the head. The 

 clear space {a}) beneath the outer wall of the tentacles corresponds to the axial 

 layer of these organs; and the interior mass {a) is the inner wall not yet hol- 

 lowed out for the chymiferous cavity. 



By the time the tentacles have grown to an elongate triangular shape {Fig. 

 23, b), and equal in length to one third the breadth of the discoid body («), their 

 axial layer {Fig. 23", a}) has its characteristic douVjle row of large cells {a} a^) ; 

 and they have considerable flexibility, not only up and down, but laterally {Fig. 23). 

 After the tentacles have grown a little longer, and assumed an oblong shape 

 {Fig. 13, e^), the centre of the internal wall of the body becomes hollowed, and 

 a large circular cavity {^) appears. The breadth of this cavity is about one half 

 that of the body, but, in consequence of the flattened shape of the latter, the 

 depth is much less. The young hydroid is concave on the side next the pro- 

 boscis, and on the opposite side convex, and the tentacles (e^) are curved slightly 

 downward. For a considerable length of time after this, there is nothing new 

 added to the organism, but the various parts of the body assume other proportions; 

 the convexity of the body increases more and more {Fig. 12, e^ e^), the tentacles 

 (e^) become longer, more slender, and, as a general thing, curled under, toward the 

 concave side of the body. By the time the tentacles {Fig. 14, e) have become 

 globular at the tips {g), and three times as long as the diameter of the convex 

 part of the body (c^), and the latter has grown deeper than broad, its concave 

 side has developed a large, broadly oval hernia (e), which projects in a straight 

 line from the centre. Its walls (c) arise directly from the bases of the tentacles, 



