588 A. WILLBY. 



gut with the inyosepta, in other words, a partial segmentation 

 of the ventral mesoderm. 



The raetapleural folds closely resemble the corresponding 

 structures in the larva of B. lanceolatum; the right fold is 

 larger than the left (especially in A. v aid i viae), and reaches 

 farther forwards. Goldschmidt finds, in general accord with 

 previous observations on Branchiostoma by MacBride 

 (1898) and van Wijhe (1902), that in Amphioxides the 

 pterygocoel or metapleural lymph-space communicates in front 

 by a fine opening with the general body-cavity, on the left 

 side behind the mouth, on the right side in front of the 

 mouth. Posteriorly the metapleural folds terminate freely, 

 directly behind the last gill-slit, and are thus independent of 

 the median ventral fin as in Branchiostoma. Goldschmidt 

 says that they cannot serve as equilibrating organs in 

 Amphioxides on account of their ventral position and 

 asymmetry ; he thinks they ai"e gill-covers and can become 

 turgid by fluid-pressure, thus approximating together and 

 closing under the gill-clefts, synchronously with the respi- 

 ratory movements. This view, however, is not confirmed 

 by observations on the living larvte of Branchiostoma. 

 Another suggestion made by Dr. Goldschmidt may conve- 

 niently be mentioned in this place. He says (p. 32) that in 

 life the lateral folds of the pharynx (plicae limitantes) 

 may be capable of being pressed together and so prevent 

 food from falling into the pars respiratoria. I think this 

 is highly improbable, and our author seems to overlook the 

 circumstance that the ingestion of food and the respiratory 

 current in Acraniata are alike effected by ciliary action. 



From the arrangement of the branchial musculature which 

 is described in detail, Goldschmidt deduces a mechanism of 

 breathing by expansion and reduction of the body-cavity, 

 analogous to lung-breathing. As I have stated above, how- 

 ever, the respiration of Branchiostoma, like that of Asci- 

 dians, is primarily promoted by ciliary currents, not by 

 muscular contraction. The branchial muscles serve only for 

 the protection and regulation of the branchial apparatus. 



