24 ON THE STEUCTUEE OF CERATELLA EUSCA (GRAY). 



cross thill coiiiiectiiig strands of the skeleton. E. external layer of ectoderm. 

 Kct. general ectoderm. G. gastrozooid. Hy. hydrophore. Sk. general skeleton. 

 Outline drawn with Camera, Zeiss A*, oc. 2. 



Fig. 12. — Porlion of a longitudinal section of a branch from which arise two 

 gonophores. The latter are cut in longitudinal section. E. external layer 

 continuous with the ectoderm of the gonophore. End. eiidoderm of the manubrium. 

 Ect. ectoderm. Ect'. ectoderm of sub-umbrella layer of the medusa. G. 

 gastrozooid. M. point at which the ectoderm dips in corresponding to the mouth 

 of the medusa. R. radial canals. Outlme drawn with Camera, Zeiss C, oc. 2. 



Fig. 13. — Longitudinal section of a branch from which another small one arises. 

 In this specimen the skeleton has more than usual a more or less definite 

 arrangement into ribs which run parallel to the length of the branch and are 

 connected liy transverse bands. Outline drawn M'ith Camera, Zeiss A*, oc. 2. 



Fig. 13a. — A small portion of the small branch in ligure 13 more highly 

 magiiilied to show the single ccenosarc tube passing along the centre. The ectoderm 

 is irregular. Letters as before. Drawn under Zeiss F, oc. 2. " 



Fig. 14. — Portion of a terminal brnncli devoid of zooids. Up the centre runs 

 a single tube with an internal unicellular layer of endoderm and an irregular ectoderm. 

 The endoderm gives off hollow processes which at certain parts (x) come into direct 

 contact with the external ectoderm (£). Further growth of these will probably give 

 rise to the gastrozooids the external layer of the colony thus forming their ectoderm. 

 Drawn under Zeiss E, oc. 2. 



