ANATOMY AND PHYSIOLOGY 



the stem being erect, while those on the membrane are 

 prostrate, (a) represents one of these new form of internal 

 defensive spiculum X 175 linear and (5) a small portion of 

 the surface of the Fucus showing its cellular structure 

 X 400 linear. 



In Hi/merapltia steUifera, Bowerbank, an exceedingly 

 thin coating British sponge, the internal defensive spicula 

 present a singular variation from the normal form. In 

 this case they assume the shape of an ordinary Florence 

 oil flask, with a somewhat elongate neck, and having a 

 beautiful star-shaped apex in place of a stopper. They 

 occur in considerable quantities ; their large bulbous bases 

 are firmly attached to the strong basal membrane of the 

 sponge, and they are projected thence at every possible 

 angle upward into the interstitial spaces. Their apices 

 are crowded with stout acutely conical spines, which 

 radiate in all directions. Fig/ 730 a, Plate XXXIV, 

 represents a group of these spicula in situ, elevated by 

 a grain of sand beneath the basal membrane ; and Fig. 34, 

 Plate I, one of the same form of spiculum, magnified 260 

 linear. In this form of spiculum, as in that of Ilymeni- 

 acido/i Cliftoni, their purposes seems to be the infliction 

 of laceration, rather than that of destruction by deep 

 wounds. In another species of Hymeraplua, H. davata, 

 these spicula have the same large bulbous bases as those of 

 H. steUifera, but their apices are acute, like those of the 

 normal forms of such spicula. In all these cases we 

 observe in their attachments the same approximation to 

 the structure of the ball-and-socket joints of the higher 

 tribes of animals, rendering them capable of yielding in 

 every possible direction to the struggles of any enemy with 

 whom they may be entangled. 



In the third division of the internal defensive spicula 

 there is an especial construction for retention as well as for 

 destruction. Their apices are usually more or less hamate, 

 as represented in Figs. 70, Plate III, and Figs. 81 and 82, 

 Plate IV, and their attachments to the sponge are usually 

 such as to alloAv of a considerable amount of flexibility 

 or motion. 



