204 HYDROID^. Part IV. 



drop from the parent stem, judging from their violent and rapid contractions and 

 the smallness of their peduncle {Fig. 12, d), but they have never been found 

 swimming freely in the sea, like the perfect medusae. 



SECTION IV 



HISTOLOGY OF CORYNE MIRABILIS. 



Proles hydroidea. Adult. — The outer Avail appears, at first sight, to be a homo- 

 geneous layer, with numerous striiB (PI. XIX. Fig. 1, g g^) running lengthwise, along 

 the stem, on its surface. Below the head these striae are double {Fig. 1, g^), and 

 run together for a greater or lesser distance, and have the appearance of being 

 the outlines of closely approximated bands, which remind one of unstriated mviscle. 

 Upon closer examination, however, these strise turu out to be mere furrows, caused 

 by the longitudinal Avrinkling of this wall. In profile {Fig. 1, b P), they may very 

 readily be seen to be superficial, especially on the tentacles {Fig. 1, P, Fig. 3). 

 The cells of this wall, as seen with objectives having wide apertures (PI. XP. 

 Fig. 14 ; PL XXIIP. Fig. 12, b P c) are fully as broad as those {d e) of the inner 

 wall ; but they are far shorter, being equal in length to the thickness of the wall 

 which they constitute. They have a flat inner face, next to the interior wall, 

 and the outer ends are rounded, and each one contains a single, excessively trans- 

 pai'ent mesoblast {P), imbedded in perfectly homogeneous contents. When the 

 hydra is stretched to its fullest extent, these cells have a hemispherical shape ; but 

 upon the contraction of the animal, they become short prisms, by mutual pressure. 



In the young hydroid (PI. XX. Fig. 6, a), which affords the best opportunity 

 for the investigation of the structure of this wall, it is seen to be transversely 

 striated, in a profile of its thickness. The striae are, without doubt, the jiarallel 

 sides of columnar cells, each one of which occupies the whole thickness of the 

 wall. But with an ordinary microscope a close examination of the thickness 

 of this wall (PI. XIX. Figs. 2, b, and 4, b; PI. XX. Figs. 2, a, and 3, a}) did not 

 disclose the least trace of cellular structure, in the adult, excepting that there were 

 numerous lasso-cells in the stem (PI. XX. Figs. 2 and 3), a few scattered along 

 the tentacles, and the usual densely packed layer at their tips (PI. XIX. Figs. 1, 

 /, 2, /, and 3, /). It is well worthy of notice that, although the lasso-cells are 

 very numerous in the outer wall of the stem (PI. XX. Fig. 2, a), they are totally 

 prevented from exercising any function, such as obtains with those on the tentacles, 

 by the thick horny sheath which shuts them off from the surrounding medium. 



