FISHES 261 



kind. Short ribs can be distinguished in the trunk, but there is 

 no trace of any sternum. 



The skeleton of the paired fins exhibits many differences 

 from the supporting parts of the limbs of terrestrial Vertebrates, 

 and comparison between the two is extremely difficult. In the 

 pectoral fin there is a very simple shoulder-girdle, consisting of 

 a curved piece of cartilage running transversely and fusing with 

 its fellow in the middle line below. At the base of the free part 

 of the fin are three cartilages, followed by a number of others, 

 and these again by jointed fin-rays. In the pelvic fins the two 

 hip-girdles are represented by a simple transverse bar, while the 

 free fin is supported by a stout rod bearing a large number of 

 fin rays. 



Digestive Organs (fig. 160). The jaws are bordered by 

 numerous rows of small pointed teeth, all much alike, and 

 replaced by fresh ones during life as often as necessitated by 

 wear and tear. These teeth are not suited for chewing, but 

 for seizing such prey as small fish, crustaceans, &c., and after- 

 wards preventing their escape. Around the margins of the 

 mouth we find all gradations between ordinary placoid scales 

 and teeth, which is intelligible when we recollect that the cavity 

 of the mouth is developed as a pit on the surface of the body. 

 The lining of such a pit, or inpushing of the general surface, 

 so to speak, is evidently equivalent to skin, and teeth here and 

 elsewhere are simply more or less modified scales, developed 

 within the margins of the mouth. The tongue is merely an 

 immobile fold on the floor of the mouth. The nasal organs do 

 not possess internal nostrils as in the lunged vertebrates. The 

 cavity of the mouth passes behind into a wide pharynx, out of 

 which the spiracular cleft and gill -clefts open; and then follow 

 gullet, U-shaped stomach, and intestine opening into a cloaca. 

 There is no clear distinction, as in forms so far considered, 

 between small and large intestine. Within this intestinal part 

 of the gut is a so-called spiral valve, which is simply a pro- 

 jecting shelf winding round and round and presenting a large 

 surface for the absorption of digested food. A large liver pours 

 bile into the beginning of the intestine, and there is also a pancreas 

 opening not far from it. 



Circulatory Organs (fig. 160). Here we have the same 

 conditions as in the Tadpole before the lungs begin to be of 



