METHODS OF ATTACK AND DEFENCE 87 



reach a circumference of several feet, simulating a small 

 coral-reef. Its contorted plates are a perfect museum of 

 Polyzoa, " so numerous are the species which delight to find 

 shelter in the quiet interstices of the colony. The exquisite 

 little colonies of Crisia ehurnea are commonly found on red 

 seaweeds, or on the branches of the Hydroid Sertularia." 



In the case of Hydroids the 

 form attacked has no special 

 means of warding off the epizoic 

 form, but each endeavours to 

 outgrow the other. Warren 

 (1909) has twice found the 

 Hydroid Lafcea dispoliam sp. n., 

 growing up inside the hydranth 

 of Sertularia bidenSy and sup- 

 planting the original polyps by 

 its own. Both cases illustrated 

 in a striking manner the struggle 

 of the host to squeeze out the 

 parasite and shut it off from 

 its depredations. The term 

 parasite employed by Warren 

 is perhaps too strong ; what 

 he is describing is simply a 

 severe case of animals of similar 

 habit competing for foothold. 



The avicularia and vibracula 

 of certain genera of Polyzoa, in 

 addition to their food-catching 

 functions, are probably of ser- pj^. 6.-Avicularium of' Poly- 

 vice in picking off intruders zoan. (i) Open ; (2) closed, 

 such as the larvae of encrusting forms, and removing foreign 

 particles which might tend to obstruct the functions of 

 the colony. An avicularium is a modified Polyzoan cell or 

 zooecium with, in typical cases, a strong resemblance to a 

 bird's head, each avicularium being provided with two long 

 snapping jaws worked by powerful muscles (see Fig. 6). 

 A vibraculum represents a further modification of a zooecium , 



