io W. J. CAIRD. 



I would suggest that some of you try to fathom the depths of the 

 mystery, and give the world of science the benefit of your researches. 

 But if for the present you allow me to assume this stage, then the cycle 

 of generations is complete. As in the case of the butterfly we have 

 various stages, viz., egg, catterpillar, chrysallis, imago ; so do we have 

 an alternation of species, hydroid, medusoid, and our missing planula. 



Sometimes obelia geniculata is coloured red, the colour being due to 

 a very minute alga which covers the surface with a net-work of chain-like 

 vegetation. O. geniculata is a phosphorescent species, and the sudden 

 illumination of a forest of it on some sombre laminarian frond is a 

 truly beautiful spectacle. If it is agitated in the dark a bluish light runs 

 along each stem, flashing fitfully from point to point as each hydranth 

 lights up its little lamp. 



I have gone fully into the life history of obelia geniculata because all 

 the others are the same, or nearly so, save for the mere outward appear- 

 ance. 



Now, let us take the next group in which the calycles are sessile, 

 i.e. y have no stalks with rings which permit of movement. These sessile 

 cups take many forms. By far the commonest one in pools is sertularia 

 purnila. The cups are in pairs set one on each side of the main axis. 

 Sometimes branches are developed. You will note here the peculiar V 

 shape the cells take. If you notice you still find the body contents 

 running the whole length of the axis, and a very tiny thread linking on 

 each hydranth. Save for a slight variation in the number of the tentacles 

 the anatomy of the hydranth is the same as that of O. geniculata. 



You will observe, too, that the stalk is marked off by nodes, deeply 

 cut into the axis, so much so that one wonders why it does not break off 

 when the swirl of some fish tail strikes it. The gonothecae are of two 

 kinds, the irregularly ovate one with the tubular rim is the female, whilst 

 the male is more slender and regularly ovate. 



But another variety sertularia tenella has the calycles alternately 

 situated. The calycles are outside the stalk although attached closely 

 to it. The calycles are very much wrinkled, and if the wrinkled calycles 

 had been sunk into the stalk instead of placed upon it, our specimen 

 would be sertularia rugosa. The life history and anatomy does not vary 

 from the specimens already considered, but I would direct your attention 

 to the relatively large size of the calycles. Judging from the specimens 

 which have gone before, this fellow ought to have them only half the size 

 of what they are. 



