28 



LUCERNARI/E AND THEIR ALLIES. 



in tliis respect, with no little abruptness, at some points on the peripherj'; but, not- 

 withstanding this, it never presents more than a single slratam of cells {figs. 85-93) 

 between its outer and inner surfaces. Consequently the varying thickness is due 

 to a diversity in the depth of the cells, and not to a greater or lesser amount of 

 these, superposed one upon another. 



62. If we commence at the mouth {fig. 53, p') and trace this layer over the vari- 

 ous subdivisions of the hydra-medusa, we shall meet with the following character- 

 istics. At the edge of the lip {{/) it {n') is continuous with the inner wall (i'), and 

 is very thin and epithelioid at all times, but varies with the amount of contraction 

 or expansion of the proboscis {manubrium, AUman). From this point passing 

 backward over the manubrium, and along its butresses to the umbella {a) and thence 

 to the margin of the latter, we do not notice any marked change in the thickness 

 of the wall until we approach tlie region of the tentacles and the anchors, but 

 observe that it is here and there Avrinkled, or compressed into tubercular or ridge- 

 like thickenings by the action of the underlying muscular layer (»*). At the margin 

 intervening between the prehensile organs it passes directly into the wall (figs. 61, 

 62, /) {ectophnigma) of the posterior foce without any marked change, but at its 

 transit to the tentacles and in particular to the anchors it becomes more massive. 

 On the tentacles which lie most distally it scarcely tliickens throughout their length, 

 but where it becomes a part of their globose terminal expansions it increases in 

 depth very abruptly {figs. 33, 54, qr), so as to form full two-thirds of the radial 

 diameter of the spheroids. On the youngest tentacles {fig. 54, A, ^") it thickens 

 quite rapidly until it reaches its maximunivat their gradually expanding tips. Its 

 passage over the anchors is signalized by quite variable changes in thickness. In 

 the median furrow {fig. 47, tr) it becomes only moderately thick, but almost abruptly 

 so, while at the sides of these organs it rapidly deepens to four, five, and even six 

 times its thickness on the face of the umbella, and finally thins out suddenly, on 

 the distal side of the anchor, just as it makes a junction with the ectophragma 



63. The ectophragma (PI. n, fig. 19 ; PI. iv, figs. 46, 47, 47% 51 ; PI. v, figs. 

 52, 54, 60; PI. \i,fgs. 61, 62, 63, 64, /to/-) is the true outer wall of the body, in 

 a homological sense ; although it seems here, upon casual observation, as we have 

 limited it, to be only a part of the external envelope. Its homological limits, 

 though, are bordered by the peripheral margin of the opsophragraa on the front 

 fiice of the umbella, and it is, therefore, restricted to the posterior face and its 

 caudal prolongation, the so-called peduncle. Throughout this wide extent of 

 length and breadth it is quite smooth and does not vary in thickness, and but 

 little excels that of the opsophragma, until it enters the region which we have' 

 designated as the adherent disk {figs. 19, 46, 51,/-) of the peduncle. There it 

 rapidly attains to double or treble its previous depth, and becomes, at times, quite 

 strongly corrugated as it follows the abrupt, sharp angles of the network of 

 furrows ; but still, like tlie opsophragma, it consists of only one stratum of cells. 



64. The opsomyaplax (PI. iv, fgs. 47, 47% 48 ; PI. v, figs. 53, 60 ; PI. VI, fgs. 

 61-64 ; PI. VII, jigs. 74, 77, 82, 83 ; PI. viii, figs. 85, 90, 91, 93, m to vf) is' the 

 stratum of muscular substance which immediately subtends the opsophragma {^ 61), 



