180 Mr. A. Hancock on the Anatomy of the 
The fluid must therefore hold in suspension the products of di- 
gestion. These may be supposed to exude through the walls of 
the intestinal canal, probably from the enlarged portion of it im 
Paludicella, and perhaps also from the upper portion of the 
stomach ; and passing into this circulation will go at once to 
nourish the various organs of the animal, all of which are bathed 
with this vivifying fluid, except the tentacles, which we shall 
afterwards see, in all probability, receive blood into their interior 
for the purpose of aération. In this way, too, we can understand 
the nourishment and growth of the tunic and the maintenance 
of the buds (which germinate from it) until they are able by the 
aid of their own tentacles to procure food. In no other way can 
the development of these buds be so easily explained. The mem- 
brane in which they take their origin must either be supplied 
with the nutritive fluid in this way or by the agency of vessels ; 
but none can be discovered either in the tunic or elsewhere. The 
external cell-walls whilst in a growing state must also be nou- 
rished by the tunic, which we have seen is united to the external 
walls at the orifice of the cell. 
The respiratory function we have stated to be exercised by the 
tentacles, but there can be no doubt that all the exposed parts 
will assist in aératmg the blood. The tentacles are hollow, 
and though I could not detect any fluid within them, it is pro- 
bable that the blood finds its way into their tubular cavities 
through the basal disc; and as they are clothed with strong 
vibratile cilia which keep a constant flow of the oxygenating 
medium over their surfaces, they would appear well adapted for 
breathing organs. It is however difficult to understand how the 
oxygenation of the blood goes on when the polype is retracted ; 
for at this time the orifice is completely closed by the folding in 
of the lips of the cell, and by muscles provided for the purpose. 
Professor Allman has supposed that the tube retractors of Palu- 
dicella exercise the function also of opening the aperture when 
in this state for the purpose of admitting the surrounding fluid. 
But I have seen nothing to warrant such supposition; and in- 
deed the tentacles being then packed close together within the 
sheath, the cilia cease to vibrate, and there is no room in which 
the water can flow around them, even supposing an opening to be 
so maintained. The tips of the tentacles too of Paludicella and 
of several of the marine species when retracted are generally 
bent down in a manner to forbid the flow of any fluid whatever 
amongst them. It would therefore seem clear, that when not in 
action the oxygenation of the blood must almost, if not entirely, 
cease in these polypes, as it must do in most of the Mollusca 
when closed up in their shelly armature. 
Nervous System.—Some years ago Professor Allman discovered 
