METHODS OF ATTACK AND DEFENCE loi 



attachment to the different organs. It is thus impossible to 

 disassociate the skeletal function in the strict sense from 

 that of defence. 



The life of sponges depending upon the maintenance of a 

 continuous flow of water through the pores and out at the 

 osculum, the occurrence of a very efficient system of protec- 

 tion, in the form of myriads of sharp spicules, occasions no 

 surprise. Around vital parts such as the oscula the spicules 

 are frequently arranged in such a 

 way as to leave no doubt as to 

 their protective function. 



The name Echinoderm (Spiny- 

 skinned) speaks for itself. The 

 limy plates which are such a 

 feature of this group, sometimes 

 forming a very complete shell or 

 *' test," are not external, but are 

 covered in starfish and sea-urchins 

 by a thin layer of epidermis, often 

 to a large extent ciliated. In the 

 sea-urchins the protection they 

 afford is greatly increased by 

 numbers of long spines articulated 

 to them. The base of a sea- 

 urchin spine is hollowed out into a 

 cup which fits over a corresponding 

 boss or elevation on the test. A 

 muscular sheath holds the spine in position and by the con- 

 traction of particular fibres can cause it to point in any 

 direction. The mode of articulation is analogous to that of 

 a ball-and-socket joint. Covering the sheath is a layer of 

 skin containing nurnerous nerve fibres (see Fig. 9). 



The chaetse of Polychaet worms are mainly used for 

 locomotion, but it is probable that they have considerable 

 protective significance as well. No attempt has ever been 

 made to explain the enormous variety of form which occurs 

 among Polychset bristles : many of them with their curious 

 processes and serrations resembling nothing more closely 



•^esir- 



Fig. 9. — Spine of sea-urchin. 

 e, epidermis ; s, calcareous 

 spine ; w, muscles ; v}, tr, 

 nerve branches. 



