ON PARAVVRIGHTIA ROBUSTA. 189 



from the stolon, and the greatest strain has to he borne. 

 The older portions of the stolon tend to be dark brown, 

 while the younger portions are pale brown or whitish. The 

 whole surface of the perisarc is generally clothed with 

 particles of mud and very fine sand (PI. XXXIII, fig. 3, ex. I., 

 and XXXIV, fig. 6, 0. L). 



The base of the hydi-anth is cup-shaped, and the perisarc 

 extends over it forming a kind of calyx (PI. XXXIII, fig. 3) ; 

 but the crown of the hydranth is not perceptibly retractile 

 into it, as in the case of Hinck's genus Atractylis. There 

 are thirteen to eighteen tentacles which spring in a single 

 whorl around the edge of the cup-shaped base. The tentacles 

 are sometimes held alternately elevated and depressed 

 (PI. XXXIII, fig. 2, J)- ^l-), ^s in the genera Perigonimus, 

 Bimeria, etc. The hypostome can scarcely be described 

 as conical, it is intermediate in shape between the conical 

 hypostome of a Perigonimus and the widely expanded, 

 trumpet-shaped hypostome of an Eudendrium. 



The hydranth can be found in three conditions (PI. XXXIII, 

 fig. 2) : (1) the ordinary contracted condition (c. p.) with 

 tentacles placed more or less vertically, covering the hypos- 

 tome ; height about 0"90 mm.; (2) hypostome and cup 

 elongated (p. el.) and tentacles alternately elevated and 

 depressed; height about 1*10 mm. ; (3) flattened condition 

 {ex. 2^.) forming a star, where the hj'postome and cup con- 

 stitute the disc, about 0*90 mm. in diameter, and the tentacles 

 form the rays. The thick perisarc of the cup must be 

 highly elastic in order to allow such very complete lateral 

 expansion. 



The endoderm of the In'dranth is red, the other parts of 

 the hydroid are translucent and white. 



The sexes are separate, and fixed gonophores are formed. 

 Both male and female gonophores have only been found 

 springing singly from the stems carrying the hydranths 

 (PI. XXXIII, fig. 2., (/, y.g.), they have not been seen 

 originating from the stolon. 



(2) Histology. — Perisarc. — The perisarc of the stolon is 



