Polyps, Jelly-fishes and Sponges. 75 



compressed, and its free extremity is pro\dded with a large 

 opening ; its general appearance is, therefore, not unlike 

 that of a polyp without tentacles, but we must note, however, 

 that the opening is not a mouth — no food passes in through 

 it ; on the contrary, it is an exhalent aperture for the 

 exit of the water current taken in through the tiny surface 

 pores, which are only to be seen under strong magnification. 

 The allied Sycon ciliatum is also abundant, but prefers deeper 

 water, and is only occasionally found between tide-marks. 

 It differs from S. compressum in being perfectly cyhndrical 

 in shape. The bodies of these two sponges are supported by a 

 complicated skeleton composed of little calcareous spicules, 

 or hard rods, which give them their characteristic felt-like 

 texture ; spicules too small to be seen with the naked eye, 

 except in the neighbourhood of the exhalent opening, where, 

 especially in S. ciliatum, they are enlarged and form a con- 

 spicuous little tuft. These two examples are among the simplest 

 members of the group. Most of our other common sponges 

 do not show any such definite structure, but form irregular 

 masses, often encrusting rocks, shells, or other appropriate 

 objects. A sponge of the latter type is the common Hali- 

 chondria panicea, the Bread-crumb Sponge, forming bro^vn or 

 orange-coloured sheets which adliere closely to rocks exposed 

 at low tide. Its consistency is approximately that of a piece 

 of bread, and the surface is studded with numerous crater-like 

 orifices, each of which corresponds to the single opening at 

 the free end of a s^'con. Like this Sponge, the Bread-crumb 

 Sponge has a skeleton of spicules, but among the members 

 of the group there is a considerable diversity in the nature 

 and composition of the skeleton ; in many, e.g., the familiar 

 Bath-sponge (a native of the Mediterranean and other warm 

 seas), spicules are absent and replaced by a meshwork of a 

 flexible homy substance. 



The Sponge figured in Fig. 91 (SipJwchalina oculata), 

 which occurs only in deep water, but is often washed ashore 

 after storms, differs from the commoner littoral species by its 

 handsome arborescent habit. 



The only other Sponge we will mention is Cliona celata, 

 interesting because it is able to bore into rocks and shells, \vith 

 a marked partiality for oyster-shells, which are often found 

 perforated by this creature. It is considered as a serious pest 

 by the oyster- growers. 



Although microscopic organisms are outside the scope of 

 this work, we cannot conclude wthout a word as to those which 



