M. C. Mereschkowsky on the Hydroida. 327 
Oorhiza borealis, nov. sp. (Pl. XV. figs. 7-11.) 
Trophosome.—The continuous layer of the hydrorhiza is 
furnished with fine spines in the form of elongated cones. 
The body of the hydranth, of a pale rose- or flesh-colour, has 
the form of an elongated cylinder. The number of tentacles 
varies from six to ten; their length is not equal, in consequence 
of different states of contraction. 
Gonosome.—The gonozooids are placed very close together 
and in great numbers, and in consequence of their spherical form 
give the surface of the hydrorhiza a tuberculose aspect. Hach 
gonozooid consists of a short spadix rising directly from the 
hydrorhiza, and a single ovum placed at the extremity of the 
spadix. 
: Locality.—The neighbourhood of the island of Solowetzky, 
at a depth not greater than 10 fathoms. 
The spadix widens at its upper extremity ; and it is upon 
this dilated part that the ovum is placed, as if upon a plate 
(fig. 8). A single spadix never bears more than one ovum, 
which may be of different sizes, sometimes very considerable, 
which proves that the ovum may grow up to a certain point— 
after which the absorption of nourishment changes the process 
of growth into a process of multiplication; the segmentation 
of the ovum commences. In the granular contents of the 
ovum a pale nucleus is always observed, and frequently a 
nucleolus. It would appear from M. Wagner’s drawings that 
the ovum is surrounded by a layer of ectoderm (Pl. XV. fig. 8), 
the same ectoderm that covers the spadix, so that the ovum is 
placed between the ectoderm and the endoderm. ‘The number 
of tentacles is very variable; but the numbers most frequently 
met with are those produced from 2, such as 6, 8, and 10, 
which leads us to regard 2 as the fundamental number of the 
Hydroids (fig. 7). 
The tentacles of Oorhiza borealis present facts of ver 
great importance. ‘Their surface at the end (Pl. XV. fig. 11) 
is not smooth ; it is mamillated, and the mamille give origin 
to something like secondary tentacles, or, rather, like pseudo- 
podia. ‘They consist of short, but not very fine, colourless, 
transparent, structureless cylinders, which spring from the 
surface of the mamille usually in groups of three or four 
together. These pseudopodia move very slowly; and M. N, 
Wagner has seen them issue and disappear just as in the 
Amebe. Moreover he has seen issuing from the surface, but 
also very slowly, larger and thicker protuberances, which be- 
came more and more rounded and inflated, and at the same 
time became constricted, so that they remained attached to 
