iv INTE OD UGTOR Y NOTE. 



indirect evidence that they at one time existed in the ancestors of 

 salpa, which were in this respect, pyrosoma-like. 



The muscle bands of salpa are easily intelligible as modified oral 

 and atrial sphincters, and they are distinctly more irregular in the 

 young than they are in the adult. In the young aggregated Salpa 

 cylindrica, the fourth and fifth body muscles are clearly seen to 

 arise as branches from an atrial sphincter, and some of the body 

 muscles arise in the same way in the aggregated Salpa pinnata. 



The peculiar anatomical relations of the pharynx and atrium of 

 ascidians are generally and justly regarded as modifications which 

 were gradually added on to the primitive tunicate type, as adapta- 

 tions to a sedentary life. If salpa has been evolved from a swim- 

 ming ancestor like appendicularia through an uninterrupted series 

 of free pelagic stages, we can give no explanation whatever of its 

 ascidian type of structure, while this is perfectly intelligible on the 

 view that it is a modified ascidian. 



W. K. Brooks. 



