180 W. K. BROOKS. 



since, as I shall show further on, it is accepted by Dohrn (Studien, 

 etc., IX, 417), who has proved himself a most rigorous critic of the 

 logic of morphology. 



There is reason to believe that the multiplication of gill-slits in 

 the tunicates has not only taken place independently, but that it 

 has taken place in a peculiar way. Anatomy and embryology give 

 evidence that while the perforations of the tunicate pharynx multi- 

 plied, the perforations of the outer M^all of the body did not ; and 

 that the external portions of the two primary clefts became distended 

 into a pair of sjmcious perithoracic chambers, each with numerous 

 ciliated openings into the pharynx, and a single opening to the 

 exterior which perhaps became enlarged as the gill-slits multiplied. 



So long as the primary function of the first pair of pharyngeal 

 clefts, the discharge of the superfluous water, was the only one^ 

 they probably remained circular like those of appendicularia ; but 

 as they became concerned in respiration and increased in number, 

 and were furnished with definite, blood-vessels, they became elongated 

 vertically and, forming a series side by side over a considerable area 

 on each side of the pharynx, they thus became much more efficient 

 organs for the aeration of the blood. 



In this simple way metamerism, that fetish of the morphologists, 

 was established among the tunicates, and there is no evidence that 

 it has ever involved any of their organs except the gill-slits and the 

 pharyngeal blood-vessels. 



A vertical series of slits, elongated longitudinally, would un- 

 doubtedly have permitted the water to escape just as well as a 

 longitudinal series elongated vertically, but it is possible that, during 

 the gradual establishment of the respiratory circulation, those of the 

 irregular and variable blood-spaces which were most nearly trans- 

 verse to the current of water from the mouth to the primary clefts, 

 were the ones which were first made definite by natural selection, 

 and that the arrangement of the gill-slits was thus determined. 



We can only conjecture how this unknown ancestral swimming 

 organism first became fixed, but the discovery of its descendants 

 on the modern sea-floor is among the possibilities of future expla- 

 nation. 



The sedentary habit undoubtedly came gradually, and at first it 

 may have been temporary, confined perhaps to the breeding season, 



