Evolution of Fishes from Invertebrates 71 



ten, to seven, and even to six pairs are now traced, evidently 

 in a steadily condensing and reducing series. In the second 

 type a series of seven (possibly more), to six, to four cleft- 

 fissures seem to have originated in the perioesophageal 

 region, from the oesophagus outward. This became the 

 forerunner of the plan shown in the gnathostome series of 

 fishes with their gill arches and gill filaments. 



The gut or post-oesophageal tract may either be a 

 simple straight tube, or frequently in metanemerteans it 

 may be expanded into irregular swellings or even into 

 paired lobes (55:93). In this manner the digestive and 

 especially the absorptive surface is increased. The same 

 result is secured in many fishes by formation of the spiral 

 valve. 



VII. The liver. 



The progressive continuity in evolution of the liver 

 from nemerteans to fishes is we believe admirably illustrat- 

 ed. In simpler metanemerteans there seems to be no special 

 appendage to the gut. But in Geonemertes Dendy says : 

 "just where it joins the oesophagus the gut gives off as usual, 

 a characteristic diverticulum which runs forward beneath 

 the last portion of the oesophagus and ends blindly." 

 Sections of this structure are seen in figure 2. But while 

 Dendy states as above, his figure might suggest that it origi- 

 nated from and emptied into any part of the "gut" region. 

 A more minute study of it is desirable. In Amphioxus 

 a like position, relation and development occur. "In 

 Petromyzon, Lepidosteus, and a few teleosts the liver re- 

 mains unilobed" also. But even in myxinoids and in higher 

 fishes it becomes a bilobed or trilobed structure that often 

 attains large size, and may develop an accessory gall- 

 bladder. 



We are still ignorant of the nature of the secretion 

 in nemerteans and Amphioxus^ but the early origin of this 

 gland as a primitively simple anterior outgrowth of the 

 foregut from nemerteans to teleosts is fairly diagnostic. 



The termination of the alimentary canal in the anus 

 agrees in all of the above. 



