92 Mr Dalyell on the Propagation of Scottish Zoophytes. 



leaves, and four organs prominent on the convexity of the body 

 appear at its base. When free the animal swims by jerks, or 

 leaps through the water, or drops gently downwards; it is in- 

 vited to move by the light, and it has survived at least eight 

 days. Then it disappears, at least I have not been able to pur- 

 sue its history longer. No other product has ever issued from 

 the vesicles of the Sertularia dichotoma Fig. 1. enlarged. 



IV. The only mode of propagation definitely ascertained of the 

 Hydra tuba, the largest of the Scottish hydrae proper, is by the 

 gemmation of the young from the body of the parent, and this is 

 gemmation in the correct acceptation of the word. I kept a co- 

 lony of these animals and their descendants during six years : 

 numbers attained maturity ; they fed rapaciously, grew and bred, 

 succeeding at all seasons of the year. But, in February 

 and March, the face or disk of some hydrae is invested by a pen- 

 dulous flexible prolongation of an inverted conical form, obliterat- 

 ing the tentacula entirely. The apex being connected with the 

 disk, this pendulous mass extends two or three lines in the course 

 of time, and is gradually developed in twenty or thirty succes- 

 sive strata gradually broadening outwards. When more ma- 

 ture, the vehement clasping of extending arms at the extremity 

 denotes, that each stratum is an animated being, which, after 

 excessive struggling, is liberated, to swim at large in the water. 

 This, also, may be associated with the Medusariae. It is consi- 

 derably larger than the preceding, two lines in diameter ; of a 

 whitish colour tending to transparence. The body resembles a 

 flattened watch-glass ; the margin dilating into from five to twelve 

 horizontal broad flattened lobes ; each cleft half-way down the 

 middle, and with a black glandular looking speck in the centre of 

 the fork. A crest resembling a quadrangular-clustered column 

 rises from the convex surface of the body, and four organs may be 

 sometimes observed on the same surface near its base. Motion is 

 accomplished in jerks or leaps, somewhat as by the Medusas pro- 

 per, from percussion of the lobes on the water, the crest being 

 downwards. Whether the pendulous mass or its individual parts 

 be contained in one common involucrum, or in many specific inte- 

 guments, is uncertain, but each of the animals composing it 

 comes successively to maturity and departs, As the pendulous 



