186 JOHNS HOPKINS UNIVEKSITY MORPHOLOGICAL MONOGRAPHS. 



cate a structure, either radially, bilaterally or serially, is a result of the 

 method of growth by cell multiplication, and that in the case in question 

 the serial reduplication has been fixed and preserved by natural selection 

 on account of its value in respiration. 



The context shows that I also regard the gill-slits of vertebrates and 

 those of tunicates as homologous structures inherited from a common 

 source, the primary pharyngeal clefts ; but that I regard the increase in 

 their number as a secondary change which has occurred in both lines 

 after their genealogical paths had diverged. 



It does not seem necessary to defend the thesis that the number of 

 gill-slits in the ascidians is the result of secondary multiplication, 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 multiplied, the perfora- 

 tions of the outer wall of the body did not ; and that the external por- 

 tions of the two primary clefts became distended into a pair of spacious 

 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 con- 

 cerned 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 undoubtedly 

 have permitted the water to escape just as well as a longitudinal series 

 elongated vertically, but it is possible that, during the gradual establish- 

 ment of the respiratory circulation, those of the irregular and variable 



