SEA-MATS AND SHELLY COEALLINES. Y5 



into the neck. By watching the contraction of these, 

 you will be enabled to determine the use of the mem- 

 brane-covered aperture up the front of the cell. At the 

 moment of the retraction of the viscera into the cell, a 

 large angular membrane is forced outward from the 

 front side, which is protruded more or less in proportion 

 to the degree of withdrawal of the polypide ; and as the 

 latter emerges again, the membrane falls back to its 

 place. It is evident, then, that this is a provision for 

 enlarging the cavity ; the walls are horny, and probably 

 almost inelastic ; but when the stomach forces the in- 

 testine forward, and the thick gullet is bent outward by 

 the withdrawal of the neck and tentacles, the needful 

 room is provided by the bulging out of this elastic mem- 

 brane, which recovers its place by the pressure of the 

 surrounding water, when the pressure of the fluids 

 within is removed. 



Now, after watching these movements of the poly- 

 pides, and the various structures whose forms and limits 

 those movements reveal, it will become manifest to you 

 that there is no visible organic connexion between the 

 animal distinctively so called and the curious bird's 

 head. This latter has a muscular system of its own, by 

 means of which its energetic motions are performed ; 

 but it appears quite isolated on the outside of the cal- 

 careous cell, and wholly cut off from the interior by 

 the knob on which it works, and by the thickness of the 

 cell-wall. Both knob and wall appear quite imperfo- 

 rate ; and yet we cannot but presume that some con- 

 nexion exists, perhaps through the medium of an ex- 

 cessively delicate and subtile, but living tissue, which 

 may be presumed not only to line, but also to cover the 

 strong cell ; just as the strong envelope and spines of 



