DEVELOPMENT OF PLANULA IN A PLUMULAKIAX HYDROID. 85 



fig 1, A). The perisarc is not so distinctly divided into an 

 onter diffuse layer and an inner compact layer as in some 

 species of Plum ularia. 



Diaiiieter of hydrorhiza about 0'19 mm. ; thickness of 

 perisarc 22*7 /u. 



Hydrocaulus. — Monosiphonic, the hydrorhiza carries up- 

 right pinnate stems of various heights. 



The proximal portion of the main-stem, not beai-ing pinnas, 

 is about half the length of the distal portion carrying them, 

 and it is somewhat irregularly divided into internodes by 

 transverse nodes (text-fig. 1, B). The distal portion of the 

 stem is regularly divided by oblique nodes. Between the 

 two portions of the main-stem there are two very oblique 

 joints (text-fig. 1, B ; C, «., n.). 



All the internodes of the main-stem in the distal pinnate 

 portion carry a hydrotheca with a mesial sub-calycine nema- 

 tophore and a pair of supra-calycine ones. 



'llie pinnaj alternate, and on an ordinary stem there are nine 

 to eleven on each side. They usually carry only two hydrotheca?, 

 but sometimes there may be three (text-fig. 1, B). The pinnas 

 are borne on a short process springing from the main-stem at 

 the back of the hydrotheca (text-fig. 1, C). The proximal 

 internode is very short, bears no structures, and the upper 

 and lower nodes are transverse (w.j, n.i) ; the rest of the pinna 

 generally consists of only two alternating non-thecate and 

 thecate internodes separated by oblique nodes. These non- 

 thecate internodes carry a median nematophore. 



Frequently the pinna?, especially the proximal ones, bear a 

 pinnule which is carried on a short process springing from 

 the level of the proximal hydrotheca. It consists of a basal 

 internode (text-fig. 2, p. t.^^.) with transverse nodes (text-fig. 1, 

 C, ?/.2, "Jt-o), followed by a non-thecate and thecate internode. 



Text-fig. 1. — A (nat. size). Colony on oyster shell. By.G. Hydrorhiza 

 bearing pinnate stem with pinn* carrying three hydrotheca\ C X 53. 

 Piece of pinnate stem with male and female gonangia. D x 55. Side 

 view of female gonangium carrying nematophores at the base. 



