70 Evolution and Distribution of Fishes 



has already drawn attention ( / : 445 ) to the prob- 

 able origin of the first gill-cleft, spiracular cleft, or hyoman- 

 dibular cleft, as it has been variously called. He showed 

 that from the two ciliated depressions of some turbellarians, 

 to the two ciliated furrows of nemerteans, and thence to 

 the ciliated branchial cavities of a cyclostome, the series is 

 a progressive one. But in nemerteans the ciliated furrows 

 externally are continued into two depressions — one on 

 either side of the head — that are placed behind the velum, 

 and come into close contact with the oesophagus, while 

 anteriorly they give rise to the sacs above treated of and 

 which the writer regards as the rudiments of the chordate 

 auditory organs. 



Neglecting the latter meanwhile, the two former regions 

 are In position alongside alike the anterior oesophageal 

 wall and dilatations or even plexuses of blood-vessels 

 formed at the anterior end of the three main blood-vessels. 

 Here then are all the connections and structures for forma- 

 tion of the first pair of branchial orifices and pouches in 

 cyclostomes. For by fusion of the adjacent tissues of the 

 oesophageal wall, of blood vessels, and of the furrows, 

 currents of water could be established through the mouth, 

 gullet, preoesophagus, and furrows to the exterior. In serial 

 succession a formation of additional canals and internal 

 dilatations could arise, seeing that abundant vascular loops 

 occur in some nemerteans, close behind the anterior system 

 already described. Such a development is foreshadowed 

 in Stichostemma eilhardi {38: i), that is referred to later. 

 A series of respiratory orifices then, along either side of 

 the preoesophagus and mesooesophagus, and in communica- 

 tion with such a rich set of vascular loops as is shown by 

 the above-named animal, would form a basis for origin 

 of the respiratory system in fishes. 



Two distinct types also seem to have originated. In 

 one of these fifteen to possibly twenty pairs of external and 

 internal orifices were formed, with pouch-like respiratory 

 enlargements between, that were embedded in the meso- 

 blastic substance of the perioesophageal region. This type 

 must have characterized the primitive ancestors of the 

 Cyclostomata, for in species of Bdellostoma fourteen to 



