86 LECTURE VII. 



common produce of which circulates through the tubular cavities for 

 the benefit of the whole animal. These currents of the nutrient fluid 

 have been observed and described by Cavolini, and more recently by 

 Mr. Joseph Lister. The genera Sertularia, Campanularia, Tubu- 

 laria, &c., which form the principal subjects of Ellis's beautiful and 

 classical work on Corallines, compose the present division of the 

 compound Hydrozoa, or hydriform polypes. 



The peculiar external horny defence prevents, as I have just ob- 

 served, the exercise of the gemmiparous faculty from effecting any other 

 change than that of adding to the general size, and to the number of 

 prehensile mouths and digestive sacs, of the individual coralline. It is 

 equally a bar to propagation by spontaneous fission ; so that the ordinary 

 phenomena of generation by ova are more conspicuous in the composite 

 than in the simple Hydrozoa. At certain points of these ramified po- 

 lypes, which points are constant in, and characteristic of, each species, 

 there are developed little elegant vase-shaped sacs, which are filled 

 with ova, and are called the " ovigerous vesicles." These are some- 

 times appended to the branches, sometimes to the axillse^ of the 

 ramified coralline : they are at first soft, and have a still softer lining 

 membrane, which is thicker and more condensed at the bottom of the 

 vesicle : it is at this part that the ova are developed, and for some 

 time they are maintained in connection with the vital tissue of the 

 polype by a kind of umbilical cord. The ova undergo a certain 

 amount of development in this situation, and acquire a ciliated 

 surface. By virtue of those primitive and universal organs of motion, 

 the vibratile cilia, they detach themselves from the umbilical stem, and 

 effect their escape from the cell. Having rowed to a convenient dis- 

 tance from the parent, the ciliated bulb subsides into an amorphous de- 

 pressed mass, which shoots out its tissue in irregular rays upon the sup- 

 porting body, to form the roots of attachment, and sends upwards a 

 pyramidal process or stem, which, at a little distance, expands into 

 a hydriform polype. The supporting stem continues to ascend, 

 divides, and proceeds to develope other polype mouths, according to 

 the prescribed pattern, and finally the ova and ovigerous sacs. In 

 some species the ovigerous cell is provided with a distinct lid or 

 operculum, which defends the ova from the sea-water in their tender 

 stages of development ; then drops off, and, allowing ingress to the 

 water, occasions an increased activity in the ciliated gemmules. 

 Sometimes a small polypus is developed from the mouth of the 

 ovigerous cell, in which state they have been described by Lowen as 

 the female polypes, the smaller and ordinary food-catching and 

 digesting polypes being regarded as the males. In all the compound 

 Hydrozoa, the ovigerous sacs are deciduous, and, having performed 



