37 







*•*. 



o 





supported upon a mesogloeal lamella. In this folded region the ectoderm is rich in gland cells, 



its cilia are comparatively short and its basal 



portion is occupied by muscle fibres arranged 



in a single layer. As the siphonoglyph is 



approached the folds suddenly cease at about 



the line of attachment of the first couple of 



deuterocnemes, and here also the mesogloea 



thickens markedly. The general character of 



the ectoderm remains unchanged, however, up 



to the line of attachment of the second couple 



of protocnemes, but there the gland cells almost 



entirely disappear, the muscle fibres also vanish 



and the cilia become much longer, these 



peculiarities distinguishing the siphonoglyph 



from the general stomatodaeal surface. 



The arrangement of the mesenteries 

 has been indicated in the introductory portion 

 of this paper and in the definition of the genus, 

 but for the sake of completeness it may be 

 stated again together with certain details 

 previously omitted. The first protocnemes are 

 short (Text-fig. IX) and bear no mesenterial 

 filaments. As in P. bcnedeni they are unusually 

 thick and narrow throughout the extent of their 

 attachment to the stomatoda^um. The second 

 and third protocnemes are alike. Each is but 

 a little longer than the first and like it is 

 sterile, although provided with a mesenterial 

 filament which presents the structure usual on 

 microcnemes. The fourth protocnemes are the 

 telocnemes; they are fertile and extend to the 

 region of the aboral pole. 



The deuterocnemes are arranged in 

 quartettes, the formula for each of these being 

 B — m — ó — M. None of the deuterocnemes 

 extend much beyond midway between the lower 

 border of the stomatoda^um and the aboral 

 pole, the longer macrocnemes of the six ventral 

 quartettes ending at about this level, while 

 those of the remaining quartettes are gradually 

 shorter toward the dorsal surface. The shorter 



37 



^ 



Vh 



