376 MR. HANCOCK ON THE ANATOMY OF THE FRESHWATER 
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 membrane 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 nourished 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 aérating the blood. The tentacles are hollow, and 
though I could not detect any fluid within them, it is probable 
that the blood finds its way into their tubular cavities through 
the basal dise ; 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 oxyge-= 
nation 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. Pro- 
fessor Allman has supposed that the tube retractors of Paludicella 
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 indeed 
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 1t must do in most of the Mollusca 
when closed up in their shelly armature. ae 
