498 



POLYZOA AND COMPOUND TUNICATA. 



orders, as follows ; I. Cheilostomata, in which the mouth of the 

 cell is sub-terminal, or not quite at its extremity (Fig. 245), is 

 somewhat crescentic in form, and is furnished with a movable 

 (generally membranous) lip, which closes it when the animal 

 retreats. This includes a large part of the species that most 

 abound on our own coasts, notwithstanding their wide differences 

 in form and habit. Thus the polyzoaries of some (as Flustra) are 

 horny and flexible, whilst those of others (as Eschar a and Rete- 

 pord] are so penetrated with calcareous matter as to be quite 

 rigid ; some grow as independent plant-like structures (as Bugula 

 and Cremellaria\ whilst others, having a like arborescent form, 

 creep over the surfaces of rocks or stone (as Hippothoa), and 

 others, again, have their cells in close apposition, and form crusts 

 which possess no definite figure (as is the case with Lepralia and 

 Membranipora). A large proportion of the Polyzoa of this order 

 are furnished with very peculiar motile appendages, which are 

 of two kinds, avicularia and vibracula. The " avicularia," or 

 "bird's-head processes," are so named from the striking resem- 

 blance they present to the head and jaws of a bird (Fig. 247, B). 

 They are generally "sessile" upon the angles or margins of the 



cells, that is, are attached 



FIG. 247. at once to them, without 



the intervention of a stalk, 

 as in Fig. 247, A, being 

 either "projecting" or 

 "immersed;" but in the 

 genera Bugula and Bicel- 

 laria, where they are pre- 

 sent at all, they are " pe- 

 dunculate" or mounted on 

 footstalks (B). Under one 

 form or the other, they are 

 wanting in but few of the 

 genera belonging to this 

 order ; and their presence 

 or absence furnishes valu- 

 able characters for the 

 discrimination of species. 

 Each avicularium has two 

 " mandibles," of which one 

 is fixed, like the upper jaw 

 of a bird, the other mov- 

 able like its lower jaw ; the 

 letter is opened and closed 



A, Portion of Cellularia ciliata, enlarged ; B, one of the j)y ^WO Sets of HlUSCleS 

 "bird's-head processes" of Bugula avicularia, more highly i i 



magnified, and seen in the act of grasping another. which are S6Cn 111 the in- 



terior of the " head ;" and 

 between them is a peculiar body, furnished with a pencil of 



