110 THE OCEANIC HYDROZOA. 



vertical plate and seven on the other ; one aperture lies in the wall of the central chamber, 

 the other six at tolerably even intervals between this and the margin. Consequently, as 

 there are more than six concentric chambers, some of them must communicate with these 

 stigmata, only indirectly. 



The vertical plate of the pneumatocyst is a thin solid plate, wholly without chambers. 

 Its concentric markings are due to mere ridges on its surface. 



A character which distinguishes the pneumatocysts of Velella and Porpita from those of 

 other Phi/sophoridcB is the attachment to their distal or under surfaces of the long filamentous 

 processes containing air, discovered by Krohn, whose cavity is subdivided at intervals by im- 

 perfect partitions, whence they appear jointed. In Felella there are, according to Kolliker, ten 

 or fifteen of these "pneumatic filaments" attached to the five or six innermost chambers; and 

 this observer agrees with their discoverer in asserting that they traverse the liver without 

 branching, and end csecally in the walls of the central polypite. 



Vogt, on the other hand, states that these filaments are inserted into the centre of the 

 disc (" par quatre canaux disposes en croix''), and ramify in the liver. Leuckart (' Z. N. K.,' 

 p. 115) is uncertain as to the mode of origin of the canals, but confirms their rami- 

 fication. 



The substance of the pneumatocyst is structureless, and consists of a nitrogenous sub- 

 stance, similar to, if not identical with, chitin. (See Leuckart, ' Z. N. K.,' p. 1 14.) 



This singularly modified pneumatocyst is inclosed on all sides by the soft parts 

 of the body or hydrosoma which extend beyond its free edges, and constitute the limb to 

 which I have already referred. 



If the crest is cut off and examined under the microscope (fig. 3), the soft parts over the 

 vertical plate of the pneumatocyst will be found to be traversed on each side by a series of 

 canals, about ^^th of an inch in diameter, which radiate from the central third of the base 

 of the crest towards its free margin. The innermost of these canals are nearly straight ; the 

 outermost are more or less curved, in correspondence with the free margin of the crest. 

 From these canals branches are given off, some of which terminate in cseca, while others 

 anastomose together, so that an irregular network of vessels spreads over the whole surface 

 of the crest. Near the termination of the vertical plate of the pneumatocyst, the canals, 

 becoming wider and less marked, take on a decided blue colour, and pass into a canal, which 

 runs along the line of junction of the vertical plate of the pneumatocyst with the limb (/>^). 

 From the outer side of this canal, again, branches are given off into the limb, which run 

 obliquely towards its margin, and in their course give off wide cascal processes on each 

 . side, which but rarely anastomose.^ Arrived near the edge of the limb, the ends of all the 

 branches are connected by a blue marginal canal (/»^), which gives off numerous short cseca 

 on its outer side. The walls of all these canals are ciliated.^ 



' Vogt (p. 19) says, in describing these canals, "lis s'anastomosent nulle part entre eux ;" but 

 so far as the species I observed is concerned, this is certainly not the case. 



" They were certainly so in my specimens. M. Vogt, however, states, " Le dois remarquer que je 

 n'ai jamais pu observer aucun mouvement dans toutes ces ramificationes vasculaires, meme en observant 

 des iudividus pleins de vie qui tout eu etant places librement sous le microscope, nagaient dans I'eau." 

 p .21.) Kolliker, on the other hand, finds a cihated epithelium in all the larger canals, (p. 51.) 



