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 in 

 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 aeration. 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 aerating 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 



