268 LECTURE X. 



fish, tlie Singio, beyond the cranium, backwards beneath the dorsal 

 myocommata upon the neural arches of the vertebrae to near the end 

 of the tail, where they terminate in blind ends. The inner tunic of 

 the sacs is a delicate vascular membrane, supplied by a continuation 

 of the posterior branchial artery. The position of the palatal opening 

 of the sac, in relation to the laminre of the second and third arches, is 

 such that water can with difficulty penetrate them, and they are 

 usually found to contain air. They are not, however, the homologues 

 of the air-bladder or of lungs, though they are analogous to the latter 

 in function. By this extreme modification of the opercular gill the 

 Singio (to which the generic name Saccobranchus is given by Cuvier) 

 is enabled to travel on land to a great distance from its native 

 rivers or marshes, and, like the Cuchia, is remarkable for surviving 

 the infliction of severe wounds.* In most fishes a rich development 

 of follicles on the walls of the gill-chamber supplies the branchial 

 machinery with a lubricating mucus. 



ARTERIES. 



The first structure most worthy of notice, in connection with 

 this system, is the vascular body already alluded to under the name 

 of ' pseudobranchia.' Mormyrus, Cobitls, Silurus, Gymnotus, Murce- 

 nophis, and ISLurcena are examples of the few genera in which it has 

 not been detected. In almost all other osseous fishes it is present, 

 situated on each side of the head, in advance of the dorsal end of the 

 first biserial gill, under the form either of a small exposed row of 

 vascular filaments, like a uniserial gill (as in all Scisenoids and many 

 other Acant/wpteri, the Plet(ro7iectidcB, and the Lepidosteus {Jig. 10. 'r); 

 or, as a vaso-ganglionic body, composed of parallel vascular lobes, and 

 covered by the membrane of the branchial chamber (as in Esox, Cy- 

 prinus, Gadiis, Jig. 69. r). In both cases the vein or efferent vessel 

 of the pseudobranchia becomes the ophthalmic artery (ib. k), and the 

 choroid ' vaso-ganglion,' when present, is developed from it. The Stur- 

 geon, like the Lej^idosteus and Lepidosiren, has a uniserial opercular 

 gill, the homologue of the first so-called ' half-gill ' of thePlagiostomes. 

 But, besides this, Von Baer discovered, on the anterior wall of the 

 ' spiracular canal,' a small vascular lamellate body, which is the 

 true pseudobranchia. It receives arterialised blood by a vessel sent 

 ofli" from the vein of the first biserial gill ; which blood, after being 

 subdivided amongst the innumerable pinnatifid capillaries of the 

 pseudobranchia, is collected again into the efferent vessel of that 



