ON A FKEE-SWIMMING HYDROID. l7 



by an enormous thickening of the ectoderm, containing 

 numerous thread- cells. 



At the aboral apex of the bell is a depression, where the 

 exumbrellar ectoderm dips in to meet an outward extension 

 of the endodermal lining of the gastral cavity. This marks 

 the spot where the young medusa is attached to the stolon 

 (fig. 22, Z.). 



The manubrium (figs. 22, 25, Man.) is large, but does not 

 project beyond the mouth of the bell. Its surface is smootb, 

 and there are no outgrowths at its extremity. 



The subumbrellar cavity is, in the middle, somewhat 

 octagonal in transverse section (fig. 25), being produced into 

 four shallow per-radial angles where the ectoderm is attached 

 to the radial canals, and four deeper interradial angles where 

 it is attached to the endodermal lamella. Immediately 

 beneath the subumbrellar ectoderm cells is a layer of trans- 

 verse C' circular") muscular fibres, and the entire epithelium 

 with its musculature is thrown into transverse f olds^ as shown 

 in figs. 22 — 24. Towards the mouth of the bell the cross- 

 section of the subumbrellar cavity becomes square, the inter- 

 radial angles alone remaining. 



The gastral cavity immediately above the manubrium is 

 cruciate in transverse section, the four arms of the cross 

 being produced outwards into the radial canals, and the 

 endoderm being greatly thickened between them to form four 

 ridges. In the manubrium itself the gastral cavity is squarish 

 or irregular in section, with a variable number of longitudinal 

 endodermal ridges. 



The four radial canals present no features of special interest, 

 nor does the thin endodermal lamella by which they remain 

 connected. Near the margin of the bell they open into the 

 circular canal (fig. 22, c. can.), enlarged per- radially in the 

 tentacle-bearing arms and then produced to form the solid 

 axes of the tentacles. 



No gonads are yet recognisable, but the ectoderm of the 

 manubrium exhibits a thickening all round about the middle 



VOL. 46, ?AKT 1. NEW SERIES. B 



