THE HYDROID FAUNA OF THE WEST OF SCOTLAND 161 



ha. 



pr. m. 



reg.p. 



P- 



accidental, due to the destruction and subsequent regeneration of 

 the hydrotheca. The stem itself shows evidence of truncation and 

 regeneration. Near the 

 base of the adcauline 

 wall of the hydrotheca 

 and on its inner surface 

 is a knob of chitin repre- 

 senting, in optical section, 

 the diaphragm which 

 separates the hydrotheca- 

 .cavity from that of the 

 stem, and which is 

 bilaterally symmetrical, 

 being well developed on 

 the adcauline wall and 

 gradually diminishing as 

 it traverses the sides of 

 the hydrotheca until on 

 the abcauline wall it has 

 disappeared. 



The hydrothecre are 

 subcylindrical, much 

 deeper than broad, and 

 bilaterally symmetrical 

 along the plane in which 

 stem and hydrotheca? lie. 

 They have an almost 

 straight abcauline and a 

 strongly convex adcau- 

 line profile, and the 



margin is divided into three deep bays separated by three large, 

 equal, pointed teeth, of which one is adcauline and the other two 

 latero-abcauline. There is a three-valved operculum. 



The hydranth is of Sertularian type, with dome - shaped 

 hypostome, a large blind-sac, and with a protractor muscle attached 

 to the abcauline wall close to the margin, and a retractor muscle 

 attached to a slight projection in the adcauline wall, a short distance 

 above the diaphragm. It has 14-16 tentacles. 



Fig. I. PARASCYl'IlUS Sl.MfLEX. X 40. 



Fragment of stem with hydrothecce of Para- 

 scy pints simplex, from between Sanda Is. 

 and Ailsa Craig. b.s, blind-sac of hy- 

 dranth ; d, diaphragm ; ha, hydrotheca ; 

 /////, hydranth ; /, perisarc of old stem ; 

 pr.m, protractor muscle of hydranth ; r.m, 

 retractor muscle of hydranth ; reg.p, thin 

 regenerated perisarc of new stem. 



The gonosome is unknown. 



Dimensions : 

 Colony, height . 

 Stolon, diameter 

 Hydrocaulus, diameter 

 Distance between hydrothecre 

 Hydrotheca, length 



maximum breadth 



79 



8 mm. 



0.24-0.25 mm. 

 0.15-0.18 ,, 

 0.45-0.82 



-45--5 6 

 0.18-0.19 ,, 



D 



