NOTES. 



39 



Selachians and other fishes, to a belief in the homodynamy of 

 median and paired fins, and he therefore concluded that the 

 latter arose as differentiations from primitively continuous lateral 

 fins just as the median fins are obviously differentiated from a 

 continuous dorsal and ventral fin-fold. Thacher further suggested 

 that the original continuous lateral fins were represented in Am- 

 phioxus by the metapleural folds. He said, " As the dorsal and 

 anal fins were specialisations of the median folds of Amphioxus, 

 so the paired fins were specialisations of the two lateral folds 

 (metapleural folds), which are supplementary to the median in 

 completing the circuit of the body." 



It has recently been observed by Professor E. A. Andrews, that 

 in a new species of Amphioxus from the Bahamas, the right meta- 

 pleural fold is continued behind into the median ventral fin. 

 Subsequently I found the same condition to obtain in a species 

 of Amphioxus from the Torres Straits. 



From these observations, and from the fact that the right half 

 of the oral hood (which apparently arises in continuity with the 

 right metapleur — vide infra) is continued in front into the 

 cephalic expansion of the dorsal fin, it would appear that there 

 is a great measure of truth in Thacher's suggestion, notwithstanding 

 the fact that in the condition in which we find them in the exist- 

 ing Amphioxus, the metapleural folds do not function as fins. 

 Thacher's hypothesis has 

 also been supported by 

 Van Wijhe. 



2. (p. 10.) The accom- pjg ig. — Diagram illustrating (by a dotted 



panying diagram (Fig. ig) line) the course of the food as it passes through 



■ ,,' i -11 1 1 lU the pharvnx and intestine of Amphioxus. (After 



Will serve to il ustrate the . ^,^ , ^,;, . , 



Andrews.) 



actual course of the food The small diverticulum on the dorsal side of 



through the pharynx q{ f^-^^ox^\\^ooAx<^v-'^s<,TA^x\^^ groove of HatM 



_ r y somewhat exaggerated. The arrows behmd the 



Amphioxus, as recently coecum indicate the rotation to which the food is 



determined bv Andrews ''■ere subjected by the action of the cilia of the 



^ . intestinal epithelium. 



irom observations on 



transparent specimens from the Bahamas. The food, enveloped 

 in the mucous secretion of the endostyle, passes along the dor- 

 sal groove of the pharynx (hyperpharyngeal groove) into the 

 intestine. 



