. Bulletin of the British Museum (Natural History). EVOLUTIONARY TRENDS IN CAPITATE HYDROIDS AND MEDUSAE 469 In Tuhularia, Gronberg (1897, pi. 4, fig. i) has shown that the lamella, associated with the parenchymatous cushion, cuts off the endoderm of the tentacles (Text-fig. 17), but this seems to be a secondary development associated with the form of the cushion in Tnbulana and may not be a primary feature as believed by Kramp. Enough has been said to indicate that the supporting lamella may be found to have little significance in classification when it is investigated more fully. In the high
. Bulletin of the British Museum (Natural History). EVOLUTIONARY TRENDS IN CAPITATE HYDROIDS AND MEDUSAE 469 In Tuhularia, Gronberg (1897, pi. 4, fig. i) has shown that the lamella, associated with the parenchymatous cushion, cuts off the endoderm of the tentacles (Text-fig. 17), but this seems to be a secondary development associated with the form of the cushion in Tnbulana and may not be a primary feature as believed by Kramp. Enough has been said to indicate that the supporting lamella may be found to have little significance in classification when it is investigated more fully. In the higher Corj-Tnorphines (such as Corymorpha nutans) there is a gastric diaphragm which divides the cavity of the hydranth into an oral and an aboral Fig. 18, Diagrammatic longitudinal section of the hydranth of Corymorpha nutans (redrawn after AUman, 1872) ; it will be noted that the diaphragm is not reduced as in Tubiilaria (Fig. 17) and that the endoderm of the hydranth is continuous with that of the tentacle: , aboral chamber ; ft., blastostyle ; ec?., ectoderm ; «K(^., endoderm ; endo., endocord ; , oral chamber ; , oral tentacle ; p., parenchyma. Fig. ig. Diagrammatic transverse section of the stem of Corymorpha nutans showing parenchyma and peripheral endodermal canals (redrawn after Stechow, 1909): ect., ecto- derm; , endodermal canals; m., mesogloea; p., gelatinous perisarc; , parenchyma. chamber (Text-fig. 18) and the stem of the hydranth is filled with parenchyma except for a series of peripheral canals representing the original cavity (Text-fig. 19). Primitive Corymorphines of small size do not possess this diaphragm which is also found in a modified form in Tuhularia. This diaphragm is of great interest to students of phylogeny in Siphonophores but as regards the Corymorphines and Tubularians its origin seems to me to be linked with the large size reached in the polyp of these forms. Although it later acquired a more specialized function it must
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