THE DEVELOPMENT OF THE HYDROZOA. I35 



mals represent individuals. The Hydrozoa are not properly- 

 compound organisms, if this phrase implies a coalescence of 

 separate individualities ; but they are organisms, the organs 

 of which tend more or less completely to become independent 

 existences or zooids. A medusoid, though it feeds and main- 

 tains itself, is, in a morphological sense, simply the detached 

 independent generative organ of the hydrosoma on which it 

 was developed ; and what is termed the " alternation of gen- 

 erations," in these and like cases, is the result of the dissocia- 

 tion of those parts of the organism on which the generative 

 function devolves, from the rest/ 



In certain Z>/5ca;9^ora belonging to the group of Trachy- 

 Jiemata, a method of multiplication by gemmation has been 

 observed, which is unknown among the other Hydrozoa. It 

 mav be termed entogastrio gemmation, the bud growing out 

 from the wall of the gastric cavit}^, into which it eventually 

 passes on its way outward ; while, in all other cases, gemma- 

 tion takes place by the formation of a diverticulum of the 

 whole wall of the gastro-vascular cavity which projects on to 

 the free surface of the body, and is detached thence (if it be- 

 come detached), at once, into the circumjacent water. The de- 

 tails of this process of entogastric gemmation have been traced 

 by Haeckel ^ in Gannirlna hastata, one of the Geryonidm, 

 As in other members of that family, a conical process of the 

 mesoderm, covered by the endoderm, projects from the roof 

 of the gastric cavity and hangs freely down into its interior. 

 Upon the surface of this, minute elevations of -g-Jifth of an 

 inch in diameter make their appearance. The cells of w^hich 

 these outgrowths are composed next become differentiated 

 into two layers — an external clear and transparent layer, 

 which is in contact with the cone, and invests the sides of the 

 elevation ; and an inner darker mass. The external layer is the 

 ectoderm of the young medusoid, the inner its endoderm. A 

 cavity, which is the commencement of the gastric cavity, ap- 

 pears in the endodermal mass, and opens outward on the free 

 side of the bud. The latter, now ^tq-*^ ^^ ^^ ^"^^^^ ^" diameter, 

 has assumed the form of a plano-convex disk, fixed by its flat 

 side to the cone, and having the oral aperture in the centre of 

 its convex free side. The disk next increasing in height, the 



» I have seen no reason to depart from the opinions on the subject of 

 'Animal individuality' enunciated in my lecture published in the Annals and 

 Magazine of Natural History for June, 1852. 



2 " Beitrage zur Naturgeschi elite der Hy drome dusen," 1865. 



