414 ANATOMY OF VERTEBRATES. 



intestine, for example, lies between the right myocommata and 

 the htemal spines in the Sole. The peritoneal serous membrane, 

 which defines the abdominal cavity, extends anteriorly to the 

 pericardium, from which it is separated by a double aponeurotic 

 septum, fig. 276, o: it is continued along the back over the ventral 

 surface of the kidneys and the air-bladder, when this exists, a little 

 way beyond the anus, and is reflected upon the alimentary canal, 

 ib. d, I, the liver, I, I, the spleen, n, the pancreas, k, or its ctecal 

 substitutes, the ovaria or testes, and the urinary bladder, if this be 

 present. In many Fishes the peritoneum does not form a shut 

 sac, but communicates with the external surface, by one orifice 

 (Bra/ichiostoma, fig. 169, od, Lepidosiren, xxxiii. pi. 25, fig. 1, 

 a), or two (Lamprey, Eel, Salmon, Sturgeon, Planirostra, 

 Chimasra, and Plagiostomes, fig. 352, p, p), situated, except 

 in the Lancelet, in or near the cloaca : the membrane in the 

 neighbourhood of these orifices is beset with vibratile cilia.' 

 The peritoneal orifices give exit to the generative products 

 (milt or roe) in the Lancelet, Myxinoids, Lam25reys, Murffinidce, 

 and Salmonidaj, Ijut not in the Lepidosiren and Plagiostomes. 

 In the Myxinoids, Aramocetes, Sturgeons, Chinifer^ and Pla- 

 giostomes, the peritoneum comnuiuicates also with the peri- 

 cardium.^ 



The jaws and mouth are subservient in most Fishes to the 

 respiratory as well as the digestive functions : in the Lancelet, 

 this community of offices extends through a great part of the 

 alimentary canal, which is dilated into a capacious sac, and is 

 richly provided with branchial vessels and vibratile cilia arranged 

 along transverse linear clefts, by which the water escapes into a 

 surrounding cavity : (the arrow a extends from the pharynx into 

 the intestine in fig. 169 :) the oesophageal portion of the alimen- 

 tary canal is here seen to be longer than the whole gastric and 

 intestinal portions. In the Cyclostomes lateral diverticula are 

 derived from the oesophagus and metamorphosed into special 

 respiratory sacs, communicating by narrow canals both with the 

 oesophagus and with the external surface, fig. 315,/, m, h: in other 

 Fishes the respiratoiy apparatus is more concentrated and brought 

 more forward, so as to communicate with the pharynx, and to 

 leave the oesophagus free for the exclusive transmission of food to 

 the stomach. 



The oesophagus, fig. 279, d, is usually a short and wide funnel- 

 shaped canal with a thick, muscular coat and a smooth epithelial 



' ccxxxiv. p. aoo. 2 i.xix. pi. 8. 



