10 



PHYLUM TUNICATA (lIROCHORDA) 



8. -Branchial basket of Leptodinum Edwardsi 

 from the left (from Seeliger after Herdman). 

 es endostyle ; fr transverse bars ; ks gill slits (stig- 

 mata). 



appear that the number, both of rows and of stigmata in a 

 row, increases with the growth of the animal. Speaking 



generally, the number is 

 smallest in the synascid- 

 ians and largest in the 

 monascidians. After full 

 size has been attained and 

 development completed, 

 there is never less than 

 three rows (species of 

 Didemnum, Distoma, etc.), 

 but in large specimens of 

 Ciona intestinalis there 

 may be 250 rows, and in 

 Phallusia mammillata as 

 many as 500. The num- 

 ber of stigmata in a row may be as small as three (Distoma 

 deeratum) and as many as 500 in large specimens of Phallusia. 



The exact method of formation of new stigmata* is disputed. It appears 

 probable that in the larva a certain number of primary stigmata are 

 formed as perforations of the pharyngeal wall (three on each side in Ciona 

 according to Willey), and that it is by the division of these and not by the 

 formation of new perforations that new stigmata are developed. 



The stigmata open externally into the atrial cavity which 

 surrounds the pharynx except in the ventral middle line, and the 

 epithelium lining them is ciliated. The 

 walls of the pharynx between the trans- 

 verse rows of stigmata may be called 

 transverse bars, and those between the 

 stigmata of a row the longitudinal bars. 

 Both transverse and longitudinal bars 

 contain blood sinuses, the transverse and 

 longitudinal vessels respectively. In many 

 ascidians vascular papillae project from 

 the transverse bars into the cavity of the 

 pharynx. These may bifurcate at their 

 ends and, extending up and down the 

 pharyngeal wall, join similar branches of 



* Willey, op. cit., Q.J.M.S., 44, 1900, p. 173. Julin, Z. f. w.Z., 76,1904 

 p. 544. Damas, op. cit., 1904. 



