Chap. III. HYDRACTINIA POLYCLINA. 235 



The Gorgonioid Polyps develop first a flat, horizontal, horny layer, as a basie, 

 and this bears exactly the same relation to the Polyps as does the young, spineless, 

 horny layer to the hydroids in a colony of Hydractinia; and when the horny 

 stem begins to rise, in the form of a spine, it is still as essentially below the 

 soft mass of the Polyps as is the fenestrated spine below the uniform layer of 

 Hydractinia. The stolouic portion, in penetrating the lateral apertures of the spines, 

 and filling up their interior with its chymiferous network, does not render the 

 horny layer, in this manner of growth, any the less a foot secretion.' Something 

 like this happens with Gorgonia flabellum, and other fenestrated forms of that 

 genus of Halcyonoids. 



SECTION II. 



REPRODUCTION OF HYDRACTINIA POLYCLINA. 



There are, essentially, three modes of reproduction in Hydractinia, namely, the 

 budding of the hydroid form from the common basis, the budding of the medusoid 

 from the hydroid form,, and the development from eggs. 



The Hydroid. — As far as we have observed, the young hydroid always buds 

 from the outskirts of the colony. The inner Avail (PI. XVI. Fig. S*", h) rises per- 

 pendicularly from the common basis, in the form of a hernia {h^), and is covered 

 by an outer wall (a}) which is continuous with the iniiform layer («). In this 

 state it does not differ from the young hydroid of Coryne mirabilis (PI. XX. FUj. 3), 

 as far as the relation of its walls to each other are concerned. The only difTerence 

 between the further development of this Hydroid and that of Coryne mirabilis is, 

 that here the tentacles arise all in the same plane, forming a single row ; other- 

 wise, in their mode of origin from the two walls of the body, there is a perfect 

 similarity. The manner in which the network of chymiferous tubes is formed is 

 very simple; horizontal herniie {Fifj. 5%/) are produced, in the direction of the 

 growth of the uniform layer (c), which in time coalesce with each other, and, 

 obliterating their walls where they come in contact, form a continuous channel of 

 communication. 



The Medusoid. — The structure and mode of development of the medusoid of 

 Hydractinia is so nearly identical in all the essential features, with those of Parypha 

 crocea and Thamnocnidia tenella and spectabilis, that it would be superfluous to 

 repeat the details which are given in another chapter, in regard to the latter 

 genera. It will be sufficient to point out in what respect the medusoid of 

 Hydractinia differs from that of these other genera, and refer to the description of 



