138 A NEW FAMILY OF HYDROIDEA. 



pond to the dark curving ones in figure 6. The thin external layer with the 

 nematophores is only shown at the edges, in reality it covers the whole surface. 

 Letters as before, x 50. 



Figure 12. — A diagrammatic drawing to show the relationship of the hard and 

 soft parts as seen at the external part of a longitudinal section of a branch. A 

 gastrozooid is shown partially expanded. Hy. the walls of the hydrotheca. C. 

 coenosarcal tube. Ma. protective zooid arising from the outermost coenosarcal tube. 

 P. the thin external layer of perisarc. P '. the continuation of the latter to form a 

 collar beyond the opening of the hydrotheca. P". the nematophore. T. tentacle. 



Figure 13. — A semi-diagrammatic drawing of a hydrotheca as seen in a thick 

 transverse section of a branch very much enlarged. The walls of the hydrotheca are 

 supported by numerous extensions of the perisarcal network, and the openings into 

 the internal-lying end of the hydrotheca are shown. The perisarc tubes have 

 branched and anastamosed to such an extent that their walls form a network, and 

 the tubular structure is not so evident as it is in the more deeply lying parts. 

 Letters as before. 



Figure 14. — The basal part of a gastrozooid cut obliquely to show the strong 

 band of muscular fibres derived from the ectoderm together with the thick layer of 

 mesogloea. Ed. ectoderm. End. large granular endoderm cells. M. thick mesog- 

 Icea layer. Mus. muscle fibres, some in connection with the ectoderm cells, from the 

 basis of which they arise, others cut at a deeper level, at which their connection with 

 ectoderm cells is not seen. Possibly some have lost their connection withectoderm 

 cells. Drawn under Zeiss F. oc. 2. 



Figure 15. — A defensive zooid highly magnified. It consists of a stalk and head. 

 The former is in connection with a coenosarcal tube one wall of which, as seen in 

 longitudinal section is shown. The centre of the stalk is solid and continuous with 

 the endoderm, though no cellular structure can be determined. Outside this is a 

 thin layer of ectoderm in which nuclei are scattered. The head consists of nemato- 

 cysts at the bases of which remnants of the cells in which they have been formed 

 can be seen with their nuclei. Ect. ectoderm. End. endoderm, M. mesogloea. 

 N. nematocyst. Drawn under Zeiss apo. 4.0 mm. apert. 0.95 oc. 12. 



Figure 16. — The collar-like operculum surrounding the margin of a hydrotheca 

 seen from above. The collar is thrown into folds. Drawn under Zeiss F. oc. 2. 



Plate 20. 



Figure 6. — A terminal branch, much enlarged to show the circular openings of 

 the hydrothecee, within which the polypes are withdrawn, and also the general spiral 

 arrangement of the hydrothecas. The surface is covered with tortuous grooves, 



