528 TUBULARI.E. 



themselves are always more or less clavate at the extremity, and 

 -are capable of being completely retracted within their polypidom 

 even when this is present. It is from Polypes of this group, and 

 from some marine forms resembling the fresh-water Hydra, that 

 true Medusae are produced by gemmation ; and we shall see 

 hereafter that there are important differences in the latter, ac- 

 cording to the form of the polype-nurse from which they ori- 

 ginate. The true Tubularice, however, display some curious 

 phenomena in their mode of reproduction. Besides the repro- 

 duction by gemmation, which is common to them and all other 

 compound Polypes, they have the power of producing a number 

 of ovigerous vesicles at certain seasons, in the form of small 

 clusters of bulbs attached to the clavate head at the bases of the 

 tentacles. In each of these a vitelline vesicle makes its appear- 

 ance, but the further progress of this presents a wonderful 

 variety. Sometimes it is developed into a single embryo furnished 

 with tentacles, upon which, according to Sir J. G. Dalyell, it i 

 able to crawl about at the bottom of the water until it meete 

 with a suitable spot for its permanent abode ; when it reverses 

 its position, so as to bring the tentacles upwards, becomes at- 

 tached by the opposite extremity, and developes itself into a 

 Polype resembling its parent. The young embryo closely re- 

 sembles a contracted Hydra. In other cases the vitelline vesicle 

 becomes divided into several parts, each of which gives rise to 

 a small ciliated embryo, analogous both in its structure and mode 

 of production to the ordinary embryo of the Sertularian Polypes. 

 But sometimes the vitellus becomes converted into a globular 

 Medusiform embryo, which swims by the contractions of its 

 body, after the rupture of the capsule containing it, and either 

 becomes directly converted into a Polype resembling its parent, 

 or by the division of the vitelline mass contained in it gives 

 origin to numerous ciliated embryos of the kind above alluded 

 to. This complicated mode of reproduction is very analogous to 

 that which takes place in the following Order, and is peculiarly 

 interesting as occurring in animals so closely allied to those from 

 which many of the Medusae are produced. 



1 180. Almost all the compound Hydroida are inhabitants of the 



