512 Development of the Stigmata in Ascidians. [May 19, 



Row. 



Number 

 of 



C L lg JJLLtX uCt* 



Observations. 



1 



17 



Length (antero-posterior) of stigmata, in middle of row, 

 - 25 mm. 



2 



16 



Ditto, ditto, ditto, 0"125 mm. 



Q 



o 



ill 



TTmiT arp fyfi v\ ^vpv^pI v plnn o*£l~prl nn p la rnnnrl anrl ■ft'cp «tp r\£ 



a longitudinally oval form. Length, 0'075 mm. 



4 



3 



vyUG T61 > *l 1U.C Dulglllfibj aJUU. .LWU iUUglLllLLiJJ.clJ.lj* UV al UUco dil/ LIlc 



dorsal extremity. 



5 



2 



One extremely wide stigma, and one small transversely 

 .elongated stigma at the dorsal extremity. 



6 



s 



o 



"I The three posterior protostigmata are completely undivided. The 

 I width of the 6th is 0-55 mm. ; of the 7th, 0425 mm. ; and of the 8th 

 J 25 mm. 



The regular development of the protostigmata in Botryllus and 

 Thylacium contrasts markedly with the phenomena observed in 

 "Phallusia" scahroides by van Beneden and Julin, but it is very 

 probable that the irregularity of formation in that species is the 

 result of secondary changes. It may, I think, be safely concluded 

 that the protostigmata of Ascidians arose primitively in regular order 

 from before backwards. 



It is very significant that in the pelagic Tunicate Pyrosoma the 

 phylogenetic inferences which have here been drawn from the de- 

 velopment of the stigmata in Ascidians are exactly fulfilled. In this 

 fcrm — as I have stated in the introduction — the stigmata are arranged 

 in a single longitudinal series along each side of the pharynx, and 

 they are transversely elongated, from the dorsal surface to the endo- 

 style. They therefore resemble precisely, both in form and in 

 arrangement, the protostigmata of larval Ascidians. Moreover, it 

 appears from the recent researches of Salensky* that the stigmata in 

 the ascidiozooids of Pyrosoma arise in regular order from before 

 backwards, just as do the protostigmata in Botryllus and Thylacium. 

 These resemblances are of too important a character to be mere 

 coincidences. I would therefore submit that in Pyrosoma we have a 

 primitive type of Caducichordate Tunicata, which is antecedent to the 

 whole of the phylum Ascidiacea, and which exhibits very closely the 

 ancestral form of pharynx from which the complicated respiratory 

 organ of the fixed Ascidians has been derived. 



It would further follow that Clavelina and its allies can no longer 



* Loc. cit, pp. 5, 33. 



