244 PISCES. 



tebra, a pulsating organ which receives the blood from the delicate 

 veins of the end of the caudal fin and propels it into the caudal vein, 

 thus constituting a caudal heart, that occurs too in Muraenophis. It 

 is a true blood-propelling organ, but occupies the same position as 

 the already-described lymphatic reservoirs found in many Fishes, and 

 conjectured to be contractile. 



In Myxine the sac of the vena porta contracts rythmically, and 

 so forms a heart ; thus in the class of Fishes have heart-like expan- 

 sions been found developed in different parts of the circulatory appa- 

 ratus, and would appear to be the more necessary from the numerous 

 plexuses of vessels through which the blood has to pass. 



These plexuses, or Retia mirabilia, have been found of the most 

 complex form and arrangement in very different organs. Thus they 

 occur upon the hepatic, portal, intestinal veins, and coeliac artery in 

 the Tunny and several species of Shark ; in the choroid gland of 

 the Bony Fishes, in the swimming-bladder, and the so-called pseudo 

 or Accessory bronchia. These latter organs, which occur in most 

 Osseous Fish, were formerly, from their resemblance to the true 

 branchiae, regarded as such, although they differ completely from 

 them in structure. They are situated mostly upon the palatal por- 

 tion of the branchial cavity in front or external to the upper extrem- 

 ity of the branchiae, like which they are pectinated and provided 

 with cartilaginous strips for their support. The blood-vessels ramify 

 upon their leaflets in a regular manner, like the barbs of a feather, 

 and receive their blood, like the opercula of the gills and lingual 

 bone, through a branch which proceeds downward from the first 

 branchial vein. A second or glandular form of these accessory 

 branchiae also occurs, and consists of deep red vascular organs com- 

 posed of several lobes, not presenting the shape of gills, although 

 they are covered by the mucous membrane of the branchial cavity. 

 The lobules themselves, however, appear like small feathers with 

 cartilaginous shafts, the latter being provided with lateral leaflets. 

 Such glandular pseudobranchiae are found in the Carp, Pike', and 

 species of Gadus. 



In the Sturgeon we find two accessory branchiae : a large one 

 situated against the operculum is a true respiratory pseudobranchia ; 

 the second, very small, and situated on the anterior wall of the 

 spiricle, consists of folds and transverse lesser folds, possesses the 

 plexiform structure of the pseudobranchiae, and obtains its arterial 

 blood from a branch of the vein upon the first branchial arch, while 



