PKOTOSTIGMATA OF MOLGULA MANHATTENSIS. 



155 



ill the oozoid of Botryllus the definite stigmata were pre- 

 ceded by protostigmata, while in the blastozoid protostigmata 

 were never formed in any generation of buds^ but the definite 

 stigmata arose separately as small rounded perforations. 



The earliest stage observed by Garstang was the tetrastig- 

 matic stage. He describes (loc. cit., p. 509) the pharynx as 

 being provided with "four pairs of transversely elongated 

 stigmata, whose transverse diameters are nine times as great 



as their antero-posterior diameters i'^ey are not 



exactly of equal size, but decrease slightly in width in regular 

 order from before backwards." 



In the next stage observed " the pharynx possesses, in 

 place of the four pairs of transversely elongated stigmata, 

 four transverse rows of small stigmata on each side." Even- 

 tually a fifth row is added. Garstang further remarks (p. 510) 

 that "the primary stigmata — if an inference may be drawn 

 from their relative sizes — arise one after another in regular 

 order from before backwards, and that they are subsequently 

 subdivided in the same order." If this is so — and it seems 

 probable that it is so — we should have the following sequence 

 ill Botryllus to be compared with the table given above 

 on the preceding page : 



I 



II 

 III 

 IV 



V 



A 

 B 

 C 

 D 



E 



Having decided that protostigmata are primitive, the dis- 

 cussion becomes narrowed down to a consideration of the proto- 

 stigmata themselves. The logic is simple, but of course not 

 demonstrative. Protostigmata are primitive ; the origin of the 

 definite stigmata by subdivision of protostigmata is indirect as 

 compared Avith the direct method of independent perforation ; 

 the formation of protostigmata by abstriction is indirect as com- 

 pared with the direct perforation in regular order from before 

 backwards, which probably occurs in Botryllus; therefore 



