DR. WYVILLE THOMSON ON HOLTENIA. 
717 
The Protozoa, to which subkingdom the mass of animal life, Phizopods and Sponges, 
occupying the depths of the sea may be referred, absorb through their whole surface 
matter, organic or inorganic, in solution or in a state of molecular subdivision. They 
are therefore specially adapted to the task of freeing the sea-water of its excess of 
organic matter ; at the same time they reduce to a solid form the dissolved carbonate of 
lime and silica, and slowly through countless ages pile up the fine calcareous sediment 
which forms the amorphous matrix of beds of limestone, and accumulate the colloid 
silica which yields the cherts and flints. 
The higher invertebrate animal forms which are associated with the Protozoa at great 
depths, though comparatively scarce and of small size, represent nearly equally all the 
leading types of structure. There is, however, no difficulty as to the nutrition of these 
higher animals. They simply feed upon the Protozoa, and the conditions of temperature, 
of light, and of the aeration of the water are such as to sustain them in life, although 
in a somewhat dwarfed and imperfectly developed condition. 
In the present communication I have only referred to these considerations involving 
the general conditions of the depths of the sea, so far as appeared to be absolutely 
necessary for the illustration of the economy of the genus Holtenia. Those conditions 
will be fully discussed by Dr. Carpenter and myself in our general report on the inves- 
tigation. 
Addendum. 
Received November 19, 1869. 
On the 21st July, 1869, during the Deep-sea Exploring Cruise of H. M. S. ‘ Porcupine,’ 
the dredge brought up from a depth of 725 fathoms, Lat. 48° 50' N., Long. 11° 09' W., off 
the mouth of the English Channel, several young specimens of Holtenia Carpenteri , with 
the sponge-body from 2 to 200 mm in length. The youngest specimens (Plate LXXI. fig. 1) 
seemed only to have passed out of the condition of gemmules. The contour of these 
young sponges is more elongated and pyriform than that of mature examples. The 
external wall consists of a single series of quinqueradiate spicules, much more regular 
in form than those of the larger sponges ; the four secondary branches decussate nearly 
at right angles, and the fifth branch is quite straight, and plunges into the sponge nearly 
vertically (Plate LXXI. fig. 8, which represents one of the spicules of the outer membrane 
of the sponge, fig. 2). It seems to be afterwards, during the growth of the sponge, that 
the spicules while enlarging greatly, become distorted and irregular in the distribution 
of their rays. The smaller spicules of the sponge-body (Plate LXXI. fig. 11) are likewise 
more regular, and a few examples were met with of the very regular hexradiate spicule 
(Plate LXXI. fig. 10), a form which is very abundant in Hyalonema. 
In the youngest sponges the stellate arrangement of the outer membrane is not yet 
perceptible ; the surface is merely divided by the spreading rays of single spicules into 
mdccclxix. 5 c 
