390 ON THE ANATOMY OF AMPHIOXUS LANCEOLATUS. 



existence, or that if it at any time had them, they must have 

 totally disappeared. I am inclined to believe that the former 

 is the real state of the case, not only from the views already 

 urged in reference to the other organs in this animal, but also 

 from the consideration that if these clefts had ever existed 

 their traces would have remained. As the seventy or eighty 

 pairs of branchial ribs cannot be looked upon as true branchial 

 arches, and as we cannot suppose that any vertebrated animal 

 could have so many branchial fissures, we are driven to the 

 conclusion that the Lancelot never had at any period of its 

 existence true branchial arches and clefts, and that the ribs have 

 been developed for a special purpose — for a mode of branchial 

 respiration hitherto undescribed in the class of fishes. 



The Lancelet respires by receiving sea-water into the 

 anterior compartment of its intestinal tube — this cavity is 

 kept dilated by the elasticity of the numerous filamentous ribs, 

 and this dilatation may be increased by the action of the 

 superimposed ventral bundles of the lateral muscles. It is 

 contracted by the action of the abdominal muscle. This is a 

 mode of respiration similar to that which prevails in the 

 tunicated molluscs. It is interesting to observe that the 

 branchial membrane of the Lancelet is exactly similar in its 

 peculiar vascularity (ramifications at right angles) to that which 

 lines the branchial cavity of the molluscs just specified. 



If the branchial membrane were examined in the living 

 animal, it would undoubtedly exhibit cilia in as great abund- 

 ance as in the branchial membrane of the ascidicc, and such a 

 ciliary arrangement must constitute one of the active agencies, 

 not only in renewing the supply of water for respiration, but 

 also in conveying food to the orifice of the digestive portion 

 of the intestinal tube. As in the ascidice, the entrance of the 

 intestino-respiratory canal is guarded by filaments. The 

 hyoid filaments of the Lancelet performing the same office as 

 the filaments at the oral orifice of the ascidice, acting as a 

 sieve in preventing the entrance of foreign bodies, or of food, 



