STRUCTURE OF HALICHONDRIA I 69 



The colour of the species is as inconstant as its form, ranging 

 from green to light brown and orange. MacMiinn concludes 

 from spectroscopic work that H. panicea contains at lease three 

 pigments, a chlorophyll, a lipochrome, and a histohaematin.^ 

 Lipochromes vary from red to yellow, chlorophyll is always 

 associated with one or more of them. Histohaematin is a 

 respiratory pigment. Proof has not yet been adduced that the 

 chlorophyll is proper to the sponge and is not contained in 

 symbiotic algae. 



In spite of all this inconstancy H. panicea is one of the most 

 easily determined species. It is only necessary to dry a small 

 fragment, including the upper sur- ,,.^ 



face ; a beautiful honeycomb - like A V'*^!* 



structure is then visible on this 

 surface, and among British sponges 

 this is a property peculiar to the 

 species (Bowerbank). Whatever the 

 form of the sponge, one or more 

 large rounded apertures are always 

 present on the exterior ; these are the 

 " oscula." In the encrusting metamp 

 the oscula are flush with the general 

 surface, while in the other cases they 

 are raised on conical projections ; 

 fistular specimens carry the osculum 

 at the distal end, and the cocks- FiG.es.-Portion of the surface of 



//.jja/ucea, from dried specimen. 

 comb has a row of them along its a, natural size ; B, magnified. 



upper edge. Much more numerous l^^.^ ^^'^^ '^"^^^ P'^*^^^"' ^'"^ 

 than the oscula are smaller apertures 



scattered over the general surface of the sponge, and known as 

 " ostia." 



If the sponge be placed in a shallow glass dish of sea water 

 the function of the orifices can be made out with the naked eye, 

 especially if a little powdered chalk or ciirmine be added to the 

 water. If the specimen has been gathered after the retreating 

 tide has left it exposed for some time, this addition is unnecessary, 

 lor as soon as it is plunged into water its current bursts vigor- 

 ously forth, and is rendered visible l:)y the ]);irticles of detritus 

 that have accumulated in the interior during the period of 

 1 Jouru. Plujslol. ix. 1888, p. 1. 



