834 " THETIS " SCIENTIFIC RESULTS. 



PERISIPHONIA EXSERTA {Johnson). 



(Plate Ixxxvii., fig. 3; c/. fig. 2.) 



Cryptolaria exserta, J. V, Johnson, Quart. Journ. Micro. Sci., 

 vi., 1858, p. 130, p]. xix., figs. 3, 3a, 3b. 



Perisiphonia Jilicula, AUman, Rep. Sci. Res. "Challenger" 

 Exped., Zool., xxiii., 1888, Hydroida, p. 44, pi. xxii., figs. 

 1-4. 



Station 57. 



Trophosovie. — A single young colony, less than 20mm. in height, 

 alone represents this genus. The stem is slender and polysi- 

 phonate, with a fascicle of about ten to twelve peripheral tubes. 

 From it arise slender branches or hydroclades, lying in one plane, 

 in sub-opposite pairs which are 2-5 mm. apart. These attain a 

 length of 10 mm. and are slightly fascicled. At the base, one or 

 more peripheral tubes accompany the offshoot of the axial tube 

 from the stem, and here the hydroclade is of very small diameter ; 

 additional tubes, which arise from the hydroclade itself, strengthen 

 the fascicle and increase the diameter in the median part, 

 although towards tlie tip the number of tubes falls away again. 



The hydrothecse are borne in two opposite series on the axial 

 tubes. They are very regularly alternate in position and are 

 close-set, each hydrotheca overlapping the distal and the proximal 

 portion of its predecessor and successor respectively, on the 

 opposite side of the axial tube. They lie in one plane. The 

 bodies of the hydrothecse are cylindrical with a somewhat 

 bulging profile, and lie closely adpressed to the axial tube for 

 more than two-thirds of their length ; while the terminal portion 

 curves gently away from the stem and projects for a short dis- 

 tance through the fascicle of peripheral tubes. The aperture of 

 a hydrotheca is round and lies in the same plane as the long axis 

 of the hydroclade. Its borders are smooth and are slightly 

 everted, and there is little tendency to regeneration of hydro- 

 theca, for at most two reduplicated margins occur. The base of 

 the hydrotheca passes into a slightly bulging portion of the 

 axial tube, corresponding to a hydrothecal peduncle, and is 

 marked by the presence of a strong diaphragm which slants from 

 the outside inwards and downwards. There is a scarcely appre- 

 ciable thickening of the exterior wall where the angle due to the 

 outward bending of the hydrotheca occurs. 



Many sarcothecse are scattered on stem and branches. On 

 the external peripheral tubes, they appear to be placed at fairly 

 regular intervals ; and their structure is constant. They are 

 short, cylindrical bodies, of diameter wide as compared with 

 their length. Each is perched on a tooth-like forward-projecting 



