SPONGES, ZOOPHYTES, AND SEA-FIRS. 37 



where they once attach themselves the sponges never move. 

 They feed on minute particles contained in the water, which 

 is swept through the porous body in continuous streams. 

 Most of them bud freely, often forming large colonies which 

 spread over the rocks as lichens spread over trees. With 

 very few exceptions, they all contain a skeleton in the form 

 of fibres or sharp spicules. The colours are often variable 

 and bright, and as a little shore hunting will soon convince 

 you, sponges often have an unpleasant smell ; in this respect 

 they resemble the more beautiful sea-anemones, which often 

 give a peculiar and disagreeable odour to the dark caverns 

 in which some species love to dwell. Sponges can generally 

 be recognised by the presence of distinct pores, and the 

 characteristic "spongy" appearance of the substance when 

 torn. In some cases not a little care is required to distin- 

 guish them from certain compound sea-squirts, which may 

 contain spicules, and from Polyzoa, or sea-mats, which often 

 contain a large amount of lime, and are occasionally not 

 unlike sponges. Both sea-squirts and Polyzoa, when care- 

 fully examined, show the presence of "polypes," of which 

 there is, of course, no trace in sponges. 



Without making any attempt to discuss the classification 

 of sponges, we may briefly note the salient characteristics 

 of three common forms. 



By far the commonest sponge on the shore rocks is the 

 crumb-of -bread sponge (Halichondria panicea), which forms 

 a thick crust, often many inches square, over rocks and 

 stones in all sorts of situations. It seems to grow equally 

 well when fully exposed to light and when sheltered in dark 

 crevices, and though perhaps commonest on a flat surface 

 does also occur on various seaweeds, especially the stems of 

 Laminaria, or oar- weed, which are often completely invested 

 by the sponge. There are indeed few spots on the tidal 

 rocks where the crumb -of -bread sponge cannot obtain a 

 foothold. In the dry state it is commonly found on the 

 shore, and such dried specimens sometimes puzzle the 

 beginner by appearing much more " spongy " than the living 

 sponge as found on the rocks. This is due to the partial 

 loss of the soft parts which brings the skeleton into greater 

 prominence. In colour the sponge varies greatly; it is 

 often distinctly green, and at other times shows various 



