440 



Protostigmata in Ascidians. 



By A. G-. Huntsman, B.A., M.B., Biological Department, University of 



Toronto. 



(Communicated by Prof. A. B. Macallum, F.R.S. Eeceived March 3, — Read 



April 24, 1913.) 



CONTENTS. 



PAGE 



Grades of Protostigmata 441 



Protostigmatic Condition in various Groups 442 



Factors of Stigmatogenesis 444 



Origin of Stigmata in various Groups 445 



Summary and Literature 452 



Introduction. 



It has been shown that at a certaiu stage in the ontogeny of Ascidians 

 there is usually a definite number of stigmata or gill slits, which are elongated 

 dorsoventrally and arranged in a longitudinal series on each side of the 

 pharynx. To these the name protostigmata was given by Garstang in 1892. 

 The stigmata of the adult differ from these in being usually very numerous, 

 indefinite in number and elongated antero-posteriorly. 



In 1904 Julin and Damas made two separate proposals to base the classifi- 

 cation of the Tunicates on the condition of the protostigmata. As these 

 proposals differed rather widely from each other and from the current 

 classifications based upon the adult condition, J have taken up the question 

 as far as it concerns the Ascidians and have investigated the origin of the 

 stigmata in a large number of genera. I have been able to confirm for myself 

 very many of the observations that have been published regarding the origin 

 of the stigmata in the genera Holozoa [Distaplia], Clavelina, Oiona, Corella, 

 Ccesira [Molgula], Botryllus and Dendrodoa (Styelopsis). In addition I have 

 investigated the genera Amaroucium, Polycitor [Distoma], Sycozoa [Colella], 

 Ascidiopsis (very near Ascidiella and Phallusia, which have been studied by 

 others), Chelyosoma, Styela, Boltenia, Pyura [Cynthia pars] and Tethyum 

 [Cynthia pars]. The last three genera belong to the family Tethyidai 

 [Cynthiidse], no members of which have been previously investigated as to the 

 origin of the stigmata. 



My interpretation of the course of development in these genera tends to 

 show that the protostigmatic condition supports the classification advanced by 

 Seeliger and revised by Hartmeyer.* 



* Bronn's ' Kl. u. Ordn. des Tierreichs.' 



