230 ARTHUR WILLEY. 



to the pair of rudimentary gill-slits, called branchial diverticula, 

 in front of the first pair of true slits, of which Dohrn established 

 the existence in the young Ammoccetes, and which never 

 actually open to the exterior ; and this circumstance should 

 be remembered in connection with the atrophy of the first pair 

 of gill-slits (i. e. the club-shaped gland and first primary slit) 

 which occurs at a certain stage in the larval development of 

 Amphioxus. 



Further, the fact that the larva of Amphioxus passes through 

 a prolonged critical period, during which it possesses only eight 

 or at most nine pairs of gill-slits, is at least suggestive. 



Addendum. 



The common ancestor of the Ascidiaus and Amphioxus 

 cannot be properly imagined until further knowledge is forth- 

 coming as to the significance of the primary pair of diverticula 

 of the prsechordal vesicle and the function of the club-shaped 

 gland. 



Gill- slits were probably present in the segmented region of 

 the trunk, which have been lost by the existing Ascidians, 

 but whether one pair or several pairs is a question. 



If the club-shaped gland is admitted to be a modified gill- 

 slit, as I hope to have shown that it is, there must have been 

 at least one pair of such slits present. 



Literature referred to. 



1. KowALEWSKY. — ' Mem. Acad. Imp. des Sciences de St. Petersburg,' 7th 



series, vol. xi, 18G7. 



2. KowALEWSKY. — ' Arcbiv fiir mikro. Anat.,' Bd. xiii, 1877. 



3. Hatschek. — ' Claus's Arbeiten,' 1881. 



4. Hatschek. — ' Zoologischer Anzeiger,' 1884. 



5. Lankester. — 'Q-uart. Journ. Micr. Sci.,' vol. xxix, 1889. 



6. Van Beneden and Julin. — 'Archives de Biologie,' vol. vi, 1SS7. 



7. DouRN. — Studien No. XII, ' Mittheiluugen aus Zool. Stat, zu Neapel,' 



Bd. vii, 1887. 



8. Lankester and Willey. — ' Quart. Journ. Micr. Sci.,' vol. xxx, 1890. 



9. Bateson. — 'Quart. Journ. Micr. Sci.,' vol. xxv, 1885. 



