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thin or wanting, while the middle layer develops an elaborate capillary 

 network. Hence, in structure the wall of the posterior chamber is 

 essentially like that of the pneumatic duct in the eel, and consists of 

 a flat epithelium under which is situated a rete mirabile. The wall of 

 the anterior chamber is lined with flat epithelium, except for a small 

 area, usually on the ventral side, where is developed the red gland. 

 The structure of the red gland is shown in Fig. 7 (Opsanus tau). It 

 consists of a series of deep folds in the single layer of the primitive 

 epithelium. The folds of the red gland are deeper and more com- 

 plicated than those found in the epithelial lining of the swim-bladder 

 in the eel, and they are concentrated in a more or less restricted area. 



outl 



midl 



Fig. 5. Diagram of swim-bladder in Opsanus. ant.ch anterior chamber, epi 

 epithelial lining, inl inner layer, midl middle layer, outl outer layer, post.ch posterior 

 chamber, rd red gland, rt capillaries of rete mirabile. 



The swim-bladder just described has hitherto received scant atten- 

 tion and has previously not been recognised as a distinct morpho- 

 logical and physiological type. Moreau ('64) was perhaps the first 

 to mention a swim-bladder of this kind (Trigla hirundo). R. W. Tower 

 ('08) has given a brief account of the gross anatomy of the swim- 

 bladder in the toadfish (Opsanus tau) and in the sea robin (Prionotus 

 carolinus). Baglioni ('08) has briefly described the swim-bladder of 

 Hippocampus guttulatus and of Syngnathus acus (see also Couch, '67). 

 I have also found this type of swim -bladder in the pipefish (Sipho- 

 stoma fuscum), in the cunner (Tautogolabrus adspersus), and in the 

 tautog (Tautoga onitis); it is also present in the striped sea-robin (Prio- 

 notus strigatus), and in the gulf toadfish (Opsanus pardus). Further 

 observation will doubtless disclose other species, which should be in- 

 cluded with this group. 



Baglioni ('08) has investigated the physiology of the swim-bladder 



