POLYZOA 



prominent spike, which fits inside the upper beak when the jaw 

 snaps. A great part of the head is filled with a strong muscle, 

 whose fibres exhibit a distinct transverse striation, and converge 

 into a median tendon. The latter is inserted into the middle 

 of the mandible. The muscle serves to close the jaws, and is 

 the representative of the muscles by which the operculum is 

 closed in an ordinary zooecium. The lower jaw is opened by 

 means of a pair of muscles which are situated immediately under 



FIG. 241. Illustrating the transition from avicularia to vibracula. A, Microporella 

 dliata Pall., Scilly Is., x 62 ; a, avicularium with short mandible (closed) ; ', 

 avicularium with vibraculoid mandible (open) ; m.p, median pore ; o, ovicell : B, 

 Mastigophara dutertrei Aud., Shetland Is., x 47 ; s, sinus of orifice ; v, seta of 

 vibraculum (or vibraculoid avicularium). 



the ectocyst of the avicularium, and pass into the mandible close 

 to its hinge. 



"Within the jaws, in the region which we may term the 

 palate, is a rounded knob, which bears a tuft of delicate sensory 

 hairs, which doubtless enable the avicularium to recognise the 

 presence of any foreign body. The closure of the mouth may, 

 indeed, be instantaneously induced by touching it with the point 

 of a needle. It has been suggested that a small mass of cells 

 which bears these hairs may represent the rudiment of the 

 polypide. 



The "vibraculum" (Fig. 242) is regarded as an avicularium 

 in which the mandible has become elongated, so as to form a 



