7 6 



ORIGIN OF CHORDATES 



III. 10 



remarkable similarity of this arrangement of the pharynx in tunicates, 

 amphioxus, and cyclostome larvae, and the partial similarity in 

 Balanoglossus. 



The tunicates show us a stage in which branchial feeding has fully 

 replaced tentacle feeding in a sessile adult. But they have a larva that 

 is beyond all question a fish-like chordate. If the adult tunicate has 

 evolved from a modified lophophore-feeding creature, how has the 



Fig. 45. To show the method by which a protochordate animal might have been 



derived from an echinoderm larva such as the auricularia. 



a. Auricularia in side view; b. protochordate in side view; c. same, dorsal view. ad.b. adoral 



band; an. anus; coel. coelom; end. endostyle; g. gill-slit; Lb. longitudinal ciliated band; 



m. mouth; n.c. nerve-cord; not. notochord. (After Garstang.) 



ascidian tadpole arisen from the auricularia larva ? Garstang's auri- 

 cularia theory, first propounded in 1894, provides a possible answer. 

 As a ciliated larva grows its means of locomotion becomes inadequate 

 because the ciliated surface increases only as the square of the linear 

 dimensions, the weight as the cube. Muscular locomotion is not sub- 

 ject to this difficulty, and some of the starfish larvae actually show 

 flapping of the elongated processes, movements that presumably 

 assist them to remain at the surface. Garstang suggests that the fish- 

 like form arose by development of muscles along the sides of the 

 elongated body, the ciliated bands being pushed upwards and even- 

 tually rolled up with their underlying sheets of nerve plexus to form 

 the neural tube. The adoral ciliated band might then well be the 

 endostyle (Fig. 45). 



This theory may seem at first sight fantastic. It is necessarily 

 speculative, but it has certain strong marks of inherent probability. It 



