Chap. I. THE CORYNE MIRABILIS. 187 



is a double-walled, branching tube (PI. XVII. Figs. 9, a b, and 15, a h, PI. XIX. 

 Fig. i, a b, and PI. XX. Fig. 2, a b). The walls of the head differ very much 

 from each other in their comparative thickness, the outer one (PI. XVII. Figs. 9, b, 

 and 11, b, PI. XIX. Fig. 2, b) being much thinner than the inner one (PI. XVII. 

 Figs. 9, a, and 11, a, PI. XIX. Fig. 2, a). In young heads, however, just budding 

 out from the pedicel (PL XX. Figs. 4 and 5), the walls (PI. XX. Fig. 6, a b) are 

 more alike in thickness ; in fact, they hardly differ in this respect. The dispro- 

 portion between the thickness of the respective walls diminishes as we follow the 

 stem downward toward its base. Just below the head (PI. XIX. Fig. 4, a b) the 

 difference remains about the same as in the head itself, and is then a little irreg- 

 ular for a short distance further down, but in the main part of the stem the 

 walls are equal in thickness (PI. XX. Fig. 2, a b). It is from this position that 

 the young heads, with walls of equal thickness, take their rise. 



The outer wall of the head suddenly thins out, and diminishes in thickness by 

 one half, where it forms the exterior wall (PI. XIX. Figs. 2, b', and 3, e) of the 

 tentacles, except at the globular tip, where it becomes much thicker than below 

 (PI. XIX. Fig. 2, /), and really forms nearly the whole bulk of the spherical expan- 

 sion at this point. The inner wall (PI. XVII. Fig. 11, /, PI. XIX. Figs. 2, a^-a\ and 

 3, a b), or solid axis of the tentacles, is a lateral growth from the inner wall of 

 the head. It far exceeds, in diameter, the thickness of the outer wall, which 

 forms a sheath around it. The outermost, or apical, portion of this axis ends in 

 a narrowed, blunt point, which projects a short distance into the globular expansion 

 of the tentacle. 



Within these double walls, the chymiferous fluid of the body circulates, and 

 may be traced by means of floating granules passing in currents from the head, 

 where the so-called digestive cavity (PL XVII. Fig. 11, d) is situated, down the 

 stem, where the common circulatory channel of the whole community begins (PL 

 XVII. Figs. 9, d, 11, d\ and 15, d, PI. XIX. Fig. 4, d), and thence, throughout 

 the whole branching stem (PL XX. Figs. 2, d, 3, d, 4, d, and 5, d), to its very base, 

 and then back again. It has never been possible to trace the circulation to 

 vibratile cilia as the propelling organs. Within the digestive cavity there is, at 

 times, an exclusive circulation, limited to the space above what might be called 

 the neck (PL XVII. Fig. 11, en), where, on such occasions, the stem contracts, so 

 as to shut off, almost entirely, the communication with the lower chymiferous 

 channel. This mode of circulation takes place, most frequently, when the head 

 assumes a very distended condition (PL XVII. Fig. 6), as if to allow the greatest 

 possible extent of absorbing surface for the nutritive fluid. The whole extent of 

 the digestive cavity and chymiferous tube is lined with brownish-red granules (PL 

 XI°. Fig. 14, dd, PL XXIIP. Fig. 12, dd), more or less closely attached to the sur- 



