MICROSCOPICAL STUDIES. 39 



A true parasite, however, sometimes afflicts colonies of Syncoryne, 

 as one of the curious PycnogonidaB (sea-spiders) makes use of the 

 developing buds as incubatory sacs, wherein their larvae may develop. 

 How the ova are deposited in the Zoophyte is unknown — but as the 

 larvae are only found in young buds, it is likely that these are selected 

 as being without the hard perisarc which is present at other parts 

 of the colony and which would render an incision difficult. Probably 

 the Pycnogonid breaks a hole in the crown of the bud and introduces 

 therein the ova. The effect is to arrest normal growth and to convert 

 the bud into a " gall " — wherein the larvae live, nourished by the 

 nutrient fluid of the coenosarcal tube, a branch of which penetrates 

 the bud. In due season the larvae burst from the gall and become 

 free. 



Study XIX. — On Sertularia pumila. 



Just as Obelia geniculata forms miniature forests on the broad 

 leaves of Laminaria (oar -weed) at a horizon seldom left bare except 

 at very low tides, so another species of the Hydroidea, Sertularia 

 pumila, in favourable situations, monopolizes the fronds of Fucus 

 at a zone some feet higher. Ellis, the worthy pioneer in our know- 

 ledge of these forms, named it the " Sea-oak Coralline," an appropriate 

 name, and one suggestive of the strong, stunted and rather coarsely 

 denticulated appearance it assumes when removed from the water. 

 It seldom attains luxuriant growth ; most frequently it is barely 

 f of an inch in height and as it retains the same breadth from base 

 to apex and is hardly branched at all, it has an incomplete and 

 truncated appearance that does not make for gracefulness. 



Occasionally, however, it grows to the height of an inch and a 

 quarter and is then beautified with several branches arranged 

 symmetrically in pairs. 



In texture it is horny, owing to the perisarc being strongly 

 chitinized. Examining a living branch in water under a low power 

 of the microscope, we forget its apparently coarse nature in the 

 loveliness of the expanded polypites. They are exquisite in their 

 slenderness, and have a great power of protrusion. Equally long 

 and graceful are the hyaline tentacles, a living rosette that surrounds 

 in a single wreath the broad and button-shaped proboscis, whose 

 summit breaks into a large and mobile mouth. The cups (hydrothecse) 

 that lodge these polyps are paired, one on either side of each segment 

 or internode in the stem. Somewhat tubular in form, each cup is, 

 at the lower end, pressed to the side of the stem internode, while 

 the upper extremity is free and bends outwards, so that each pair, 

 with their stem internode, form a V-shaped figure. The aperture 



