Protostigmata in Ascidians. 



441 



Grades of Protostigmata. 



In fig. 1, C represents diagrammatically the left side of the pharynx of a 

 young Ciona. It can be clearly seen that there are two grades of protostig- 

 mata. Counting from the anterior end (left side) the first, fourth, and fifth 

 are longer than the second, third, and sixth. The former are the first to 

 appear and they perforate the pharyngeal wall independently of each other. 

 They give rise to the latter three as indicated in the figure, by their ventral 

 ends extending and turning upward. They then divide at the angles. De Selys 

 and Damas (1901) have proposed the term primary protostigmata for the 

 three arising by independent perforation and the term secondary proto- 

 stigmata for the six resulting from their division. 



But the fourth protostigma differs from the first and fifth in having its 

 concave face forward and in turning forward and upward before division 



Fig. 1. — Various types of protostigrnatic condition. The left side of the pharynx is 

 shown, the anterior end being toward the left. 



instead of backward and upward. Willey (1893), De Selys (1901) and 

 Damas (1904) have recorded instances in Ciona and Ascidiella, in which the 

 first and fourth protostigmata were continuous ventrally (before the second 

 and third had been formed). Willey concluded from this that the first four 

 protostigmata were derived from a single primary gill slit. This view seems 

 to be justified by the facts. The fourth behaves as if it had been derived from 

 the first in the same way that the sixth is derived from the fifth. The fourth 

 is equal in size to the first because it is derived from the latter at such an 

 early stage, before perforation. They are both able to repeat the process and 

 thus form four stigmata before the change occurs that brings about the 

 formation of the definitive stigmata. The fifth is able to divide once only. 



I conclude therefore that in Ciona the stigmatic rudiment or anlage divides 

 into two parts placed one behind the other. They correspond in position to 



VOL. LXXXVI. — B. 2 K 



