1913.] 



Proto stigmata in Ascidians. 



443 



Appendicularians, Salpidee, Doliolidfe and Pyrosomatidse ; in the second group 

 are the Krikobranchia and the Perophorid;e ; and in the third group all 

 the simple Ascidians. These groups are natural enough, but 1 cannot agree 

 with his interpretation of the development in many forms. Having to 

 prove that all the stigmata of the adult are derived from a limited number 

 of first stigmata, he puts a rather forced construction on many of the facts. 

 If his method were followed out fully, all the stigmata in each and every 

 Tunicate could be derived from a single pair of stigmata. I think that 

 this is a legitimate conception, but it does not lead to any divisions within 

 the group. The Appendiculariau is without doubt a primitive form. The 

 Doliolidee, Pyrosomatidse and Ascidiacese show remarkable diversity in the 

 methods by which a large number of stigmata are derived from a single 

 pair corresponding with those of the Appendicularian. It is by studying these 

 methods that we get an insight into the affinities of the various forms. Mere 

 numbers at any definite stage are of subordinate importance to the process. 



The rather definite separation of the process into two parts, (1) the 

 formation of a longitudinal series of protostigmata, and (2) the trans- 

 formation of these into transverse rows, makes it possible to institute a 

 comparison of the early stages alone. In fig. 1 I have represented 

 diagrammatically various types of protostigmatic condition, indicating not 

 only the number but also the method of origin. 



A represents the condition in the Krikobranchia, a group consisting of the 

 majority of the compound Ascidians. I agree with Julin and Damas in 

 considering that two protostigmata are represented in this group. True 

 protostigmata are never seen. The process of division into definite stigmata 

 begins before perforation, as will be shown subsequently. The two 

 protostigmata are evidently of the second order, and represent a single 

 primary protostigma. 



B represents the condition in Perophora, modified from the description by 

 Damas. Four protostigmata of the third order are present, representing a 

 single primary one. Here also typical protostigmata do not occur, and the 

 interpretation is doubtful, as will be shown subsequently. 



C represents the condition in other Dictyobranchiates (Cioua, Phal- 

 lusia, etc.). This condition has been described above. There are four 

 tertiary protostigmata, representing a single primary, and behind these 

 two secondary ones, representing another primary. These forms are peculiar 

 in possessing two primary protostigmata. 



D shows the condition in the Cyesiridse and some Tethyidse {Pyura and 

 Tethyum [Cynthia]). There are six secondary, representing three primary 

 protostigmata. 



2x2 



