THE CAMPANULAEIDiE AND THE BONNEVIELLID^. 



19 



is covered with an investment of chitin, and the interior of the cup is in close contact with 

 the ectoderm of the developing hydranth throughout, and the endoderm lines the cavity of 

 the latter. Later the lower portion of the hydranth body becomes separated from the hydro- 

 thecal wall by a sort of shrinking (fig. 67), and this part of the hydranth has an outer layer 

 of ectoderm much rhinner than that which invests the upper or distal part, which shows an 

 ectoderm composed of columnar cells in close contact with the upper part of the hydro thecal 

 wall and with the thin periderm cap. At about this time the specific ornamentation of the 

 hydrothecal margin is formed and, in the lower part of the hydrotheca, the diaphragm becomes 

 differentiated. Around the distal end of the developing hj^dranth the rudiments of the tentacles 

 are formed, the first indication being the appearance of a ring of endoderm cells around the inner 



DEVELOPMENT OF THE CAMPA^^JLAKIAN HVDRANTH. 



(After Kuhn.) 



Fig. 05. Gonothyrsea lovenl. Longitudinal section of a hydranth bud. 



Fig. 66. Obelia dichotoma. Longitudinal section of an older bud. 



Fig. 67. Gonothyrsea, loveni. Longitudinal section of a hydranth as it is beginning to separate from its hydrotheca. 



Fig. 6S. Gonothyrxa loveni. Longitudinal section of a hydranth after the budding of the tentacles and before the formation of the mouth. 



wall of the end of the developing hydranth, the cells being derived from the original endodermal 

 lining of the gastric cavity. In places these endodermal cells "pile up'' as it were into little 

 cones representing the tentacle buds and pusliing out the overlying ectoderm cells, the whole 

 appearance being, as the author says, much like the vegetation point in plants. The tentacles 

 then appear as widely separated swellings placed in a circle around the broad flattened end 

 of the hydranth, and at the same time the end itself becomes dome-shaped and the tentacles 

 become separated from the endoderm of the gastric cavity by a thin layer of stutzlamella 

 (fig. 68). Later the dome-shaped proboscis becomes sharply constricted off at the bott?om and 

 the characteristic "trumpet-shaped" jiroboscis of the Campanularidse is formed. Meanwhile 

 the tentacles themselves assume their final histological arrangement of a central core or a 

 single row of endodermal cells enveloped by a stutzlamella and ectoderm. 



Before this occurs, however, the thin layer of chitin which has covered the distal end of the 

 hydrotheca has been separated from the hydranth, so that the latter is now entirely free except 

 from the diaphragm at its base. 



As the hydranth approaches maturity, after the budding of the tentacles (fig. 47), a great 

 difl'erentiation appears in the endoderm cells. Those in the hypostome become very small and 



