1913.] 



Protostigmata in Ascidians. 



449 



or may not be formed and are never more numerous than six. This Order is 

 sharply marked off from the other two Orders. 



Inside the Order, the four methods of formation of protostigmata do not 

 correspond with the four families of the Order. The Tethyids exhibit three 

 methods and the Styelids two. But comparative anatomy has shown that 

 some of the Tethyidse approach the Csesiridag and others approach the 

 Styelidre. And in a similar way the Styelidse show affinities with both the 

 Tethyidse and Botryllidte. The protostigmatic condition therefore corroborates 

 the classification based upon the adult condition. 



There are two distinct methods in the Dictyobranchia, The most general 

 one is that described above for Ciona. The first stage is the appearance of 

 two stigmata of about the same size. From their occasional connection with 

 each other and from their subsequent behaviour, they are very evidently 

 halves of an original single element which is doubtless homologous with the 

 first stigma of the Ptychobranchiate. They are secondary protostigmata, 

 corresponding with one primary. Their probable history before perforation 

 is indicated in the figure. The original element has turned up at its lower 

 end in the usual fashion and then divided into two. 



The two secondary protostigmata subdivide into four tertiary, and at the 

 same time a second primary appears and divides into 1 two secondary ones. 

 The new stigmata are connected for a short time with the old in Ciona, and 

 this has been shown in the figure. In Ascidiopsis (and doubtless in other 

 genera as well) the intercalated stigmata are formed as has been described 

 above for Styela and Boltenia. Stigmatic material is separated from the 

 lower ends of the old stigmata, and subsequently cavities appear in the 

 new parts. This is only a slight modification of the process occurring in 

 Ciona. 



In Perophora the accounts that have been given do not agree. Julin 

 states that the stigmata arise essentially in the same way as in Distaplia, 

 but gives no figures. Darnas has figured the process and interprets it in such 

 a way as to bring it into line with the condition in other Dictyobranchiates. 

 Not having an opportunity of examining this genus, I have followed Damas' 

 account. The figures for this genus given in the scheme of fig. 2 are from 

 Damas. The two stigmata that are the first to appear evidently correspond 

 with the similar two of other Dictyobranchiates, that is, they are two 

 secondary protostigmata representing a single primary. By division and 

 intercalation four tertiary are formed. The process is repeated dorsally as 

 shown in the figure. As a result of this curious repetition the second and 

 third protostigmata are each represented by a dorsal and a ventral part 

 derived from the dorsal and ventral ends of the first and fourth. Damas 



