GAS GL.VNUS OF SOME TKLKOSTEAX FISHES. 195 



ducfcs just external to the cells of the gii,s gland. Figure 21 

 (PI. III.) shows several stages of this disintegration of the 

 erythrocytes, the significance of which I shall discuss later. 



Figure 20 represents a highly-magnified transverse section 

 through the fine arterial and venous capillaries composing the 

 mass of the i-ete mirabile. The two kinds of capillaries are 

 quite indistinguishable by mere inspection. The capillaries are 

 separated from each other by a small amount of connective tissue. 

 In the very young fish, l. e. before the capillaries have subdivided 

 to the extent found in the adult, the ai-terial ca.pillai'ies are dis- 

 tinguishable from the venous, the former being thicker- walled and 

 somewhat smaller in calibre (see also Bykowski & Nusbaum, 23). 



Thus the chief features of the second or tSyiignathiis type of 

 "red body" are that the glandular epithelium is arranged in 

 simple folds, that it is restricted in aren, not lining the whole of 

 the bladder cavity, and that the rete mirabile is contiguous with 

 the gas gland, although a small amount of reunion of the capillaries 

 of the rete may occur before these supply the epithelium. 



Other ExMiivples of the Syngnathus Ttjjje of''' Red BodyT 



GOBIUS NIGER. 



As another example of the second or Syngnathus type of 

 bladder vasculo -glandular apparatus I shall now describe that 

 of Gobiits niger caught at Naples*. The " red body " of Gohius 

 is a small oval red patch situated just anterior to the centre of the 

 oval outline of the bladder (PI. III. fig. 22). It consists of a single 

 rete mirabile which supplies a small area of glandular ej^ithelium. 

 As seen in a series of transverse sections, the artery and vein, 

 Avhich break up to form the rete, at first lie outside the several 

 layers of the bladder wall (PI. IV. fig. 23) together with large 

 masses of pancreas. The artery and vein subdivide as usual to 

 form a cluster of small arteries and veins, which in Gohnis, as in 

 Syngnathus, are intermingled with masses of panci-ea-s (fig. 24). 

 It will also be noticed that this cluster of arteries and veins is at 

 this stage in process of breaking through the thin dense outermost 

 layer of the bladder toall, so as to lie within this latter, large 

 masses of pancreas being included with the cluster of vessels. 

 Still moi-e posteriorly when the arteries and veins have undergone 

 subdivision to a greater extent, the whole mass of vessels together 

 with the included portions of pancreas have become almost 

 entirely included loithin the bladder wall, small portions of pan- 

 creas only being left outside (fig. 25). Finally, when the rete 

 mirabile is completely formed it is wholly included within the sub- 

 stance of the bladder wall, as in previous genera (fig. 26). The 

 rate mirabile supplies the adjacent area of glandular epithelium 



* Grohiiis paganellus, obtained from Plymouth, possesses a star-shaped " red bodv " 

 resembling tiiat of Perlstethus in constructiou (PI. V. fig. 37, and text-tig. 57), 

 though less definite in outline. Gohius minutus, also obtained from Plymouth, has 

 a, similar star-shaped " red bodj'," but the gas gland epithelium in some specimens 

 is hai-dh' folded at all, merely consisting of a single layer of cells oncloaing- large 

 capillaries ; in others it is folded as in Lrobitts iiiijcr (PI. IV. fig. 27). 



