x TELEOSTEAN FISHES 359 



there are one or two peculiar features to be noted. The spiral valve has 

 disappeared from the intestine, although slight vestiges of it persist in 

 a few cases. In correlation with this a secondary lengthening of the 

 intestine has taken place so that it takes a tortuous course through the 

 splanchnocoele in striking contrast with the condition in an Elasmo- 

 branch. In the region of the pylorus a varying number of glandular 

 more or less finger-shaped outgrowths of the wall of the alimentary 

 canal are found the pyloric caeca. Finally there is a well-marked 

 tendency for the cloaca the terminal part of the alimentary canal into 

 which open the renal and genital ducts to become flattened out so that 

 anus, genital and renal openings come to be situated on the external 

 surface one behind the other. 



The renal organ of the teleost is an opisthonephros. The right and 

 left organs with their ducts are at first separate but as development goes 

 on fusion takes place which may affect only the hinder portion of the 

 ducts or may spread to the hinder portion of the renal organs them- 

 selves. 



The reproductive organs show a peculiarity highly characteristic of 

 the group namely that the wall of the gonad is continued backwards 

 as a simple tubular prolongation which functions as its duct and opens 

 to the exterior by an opening common to the two ducts between the 

 anal and renal openings. The probable evolutionary origin of these 

 genital ducts of the Teleostei forms an interesting problem in morphology. 

 The general relations of the ducts are very similar in the two sexes but 

 the probability seems to be that this likeness is secondary and that the 

 ducts of the male and female have arisen in evolution in quite different 

 ways. 



In the elasmobranch the eggs are of large size and as a consequence 

 relatively few in number. In the typical teleost on the other hand the 

 eggs have become greatly reduced in size and greatly increased in 

 number. At the same time they are without the elaborate envelopes 

 secreted by the oviducal wall that are present in the elasmobranch. 

 Apparently in correlation with the fact that a long glandular oviduct is 

 no longer needed for the formation of such envelopes the Miillerian duct 

 of the teleost has become much reduced in length. The ovary has 

 developed the peculiarity that the eggs set free from its surface fall 

 not into the main splanchnocoele but into an enclosed space formed 

 either by the lower edge of the ovary becoming fused to the body-wall 

 (Fig. 150, B) or by an ingrowth of the surface of the ovary becoming 

 covered in by its edges meeting (Fig. 150, A). In either case the cavity 



