m r. mm PHYLUM MOLLUSCOIDA 341 



Hydroid Zoophytes, and only on a more minute inspection are 

 found to differ totally from the latter and to exhibit a very much 

 higher type of structure. 



1. Example of the Class. — Bugula avicularia. 



Bugula avicularia, the common Birds-Head Coralline (Fig. 276), 

 occurs in brown or purple bushy tufts, two or three inches long, on 

 rocks, piles of jetties, and similar situations on the sea-shore in all 

 parts of the world. On a naked-eye examination it presents a 

 considerable resemblance to a Hydroid Zoophyte, and might readily 

 be taken for a member of that group. It consists of dichotomously 

 branching narrow stems, which are rooted by a number of slender 

 root-filaments. Each stem is found, when examined with a lens, to 

 be made up of a number of elements, the zoaecia of the colony, 

 which are closely united together and arranged in four longitudinal 

 rows. The zocecia are approximately cylindrical in shape, but 

 broader distally than proximally, four or five times as long as broad, 

 and have, near the distal end, a wide crescentic aperture — the 

 " mouth " of the zoopcium — on either side of which is a short blunt 

 spine. A rounded structure — the ocecium — in many parts of the 

 colony lies in front of each zocecium (Fig. 276, ocec). On each- 

 zocecium, except a few at the extremities of the branches, is a 

 remarkable appendage, the avicularium (avic), having very much 

 the appearance of a bird's head supported on a very short stalk : 

 if the Bugula. is examined under the microscope in the living 

 condition, the avicularia will be found to be in almost constant 

 movement, turning from side to side ; and a movable part, com- 

 parable to the lower jaw of the bird's head, will often be seen to 

 be moved in such a way that the mouth of the avicularium is 

 opened very widely and then becomes closed up with a quick 

 " snap." All the parts hitherto mentioned can be shown, by using 

 appropriate tests, to be composed of some material akin to chitin 

 in composition. The chitinous wall of the zocecia is the hardened 

 and thickened cuticle of the zooids, having beneath it the soft body- 

 wall. 1 The anterior region of the body of the zooid forms an 

 introvert, i.e. is capable of being involuted like the finger of a 

 glove within the more posterior part : the cuticle covering this, con- 

 tinuous behind with the thick ectocyst, is quite thin and flexible. 

 When the introvert is everted it is seen to bear at its anterior end a 

 circlet of usually fourteen long, slender filiform tentacles (tent) on a 

 circular ridge or lophophore surrounding the mouth of the zooid. The 

 tentacles are densely ciliated except along their outer surfaces : the 

 cilia vibrate actively in such a way as to drive currents of water, 



1 The terms ectocyst and eiidocyat are commonly applied respectively to the 

 hardened cuticle of the zooid and its soft body-wall. 



