423 THE ANIMAL KINGDOM. 



of the intestine in the air-swallowing Loach (Cobitis fossilis}. The dorsal 

 mesentery often becomes largely fenestrated, sometimes almost absorbed. 

 A nearly complete ventral mesentery (p. 357) is found in the Dipnoi, a few 

 other Fish (Muraenoids), and a posterior rudiment in Lepidosteus. The 

 rectum ends in a cloaca in Elasmobranchii and Dipnoi common to it and 

 the urogenital ducts. The anus in other Fish lies in front of the urogenital 

 apertures. 



The liver is unilobar in Lepidosteus and a few Teleostei, bi- or tri-lobed 

 in other Fish. The two lobes are connected by peritoneum only in some 

 Rays, e.g. Torpedo. They are of huge size, reaching to the cloaca in some 

 Sharks. A gall-bladder is rarely absent, and is either imbedded in the liver 

 or lies between its lobes ; it varies much in form and size, e. g. it may 

 reach to the anus, as in Scomberidae (Mackerels). There are several bile- 

 ducts, but they usually unite into a single duct which opens near the 

 pylorus. A uni- or bi-lobed pancreas is present in Elasmobranchii : it is 

 large and lobed in Acipenser^ small in Lepidosteus (? rudimentary), and a 

 few Teleostei, e. g. Esox, but in the last named order it is generally either a 

 diffused gland or absent altogether. 



The external gill-clefts are exposed in all Elasmobranchii, and the 

 first (post-mandibular) or spiracle is always so when present. They are 

 protected in other orders by an opercular fold from the hyoid arch, in which 

 opercular bones and sometimes branchiostegal rays (p. 93) are formed 

 except in Holocephali. The spiracle is found in Elasmobranchii, in 

 Acipenser and Spatularia among chondrostean, and Polypterus among 

 bony Ganoidei. Its gill, if retained, derives arterial blood from the 

 hyoidean or opercular gill. The post-hyoidean gill pouches of the embryo 

 retain the character of pouches in Elasmobranchii, where they are five in 

 number, except in Hexanchus and Heptanchus, with six and seven respec- 

 tively ; and in Holocephali, where they are four. The septa, which are con- 

 tinued outwards from the branchial arches, atrophy more or less completely 

 in all other Fish, and thus leave the gill-filaments projecting freely, at least 

 at their apices. The respiratory tissue has the form of radiating folds 

 when the septa are complete ; of filaments supported by an axial cartil- 

 age when they are atrophied ; longest about the centre of the arch or 

 septum. The first set is borne upon the hyoid arch, or the operculum, 

 when present : hence hyoidean or opercular gill. It is found in Elasmo- 

 branchii, Holocephali, Acipenser, Scaphirhynchus, and Lepidosteus, among 

 Ganoidei, where it receives the venous blood of the first branchial artery 

 or a branch of that vein in Protopterus and Lepidosiren among Dipnoi^ : in 

 many Teleostei, where it is known as pseudobranchia (a term also applied 

 to the spiracular gill of Ganoidei}, and is supplied with blood from the 

 hyoidean artery. This pseudobranchia has the form of either a set of 



1 It is present in Ceratodus, but the source of its blood-supply is apparently unknown. 



