NORTH AMERICAN EARLY TERTIARY BRYOZOA. 61 



over this membrane. This arrangement avoids the disjunction of the zooecia with 

 rigid skeleton in consequence of the inevitable growth of the substratum. 



The species which creep over very delicate algae have some longer tuberosities. 

 They have as their object the withdrawal of the ectocyst from the cryptocyst, to 

 augment the volume, in order to diminish the weight and thus to be. able to grow 

 on the algae, without making it bend over, risking an overturn. The reader will 

 find in the course of this monograph some very curious variations of the zoarial 

 hydrostatic system which it is needless to mention here. 



Use of the vibracula. Either attached by some radicels or placed under the 

 algae, the Lunulites, notably because of their turbinate form, are in a very unstable 

 position ; they upset at the least current of water. The animal maintains its normal 

 position by the aid of long articulated filaments called vibracula. These are then 

 objects for stabilization somewhat analogous to the balancing rod of a rope dancer. 



These vibracula have a like function in man}' articulated genera ; Scrupoc.cllaria, 

 Bwellaria, Caberea, etc., parasites on algae, movements of which constantly com- 

 promise the equilibrium of their zoaria. 



FUNCTIONS OF RELATION. 



Avicularia. The avicularia are heterozooecia formed of a small cell or avicu- 

 larian cavity and of a mobile mandible. 



The avicularian cavity is constructed like one of the adjacent normal zooecia. 

 It contains an endocyst, an ectocyst, a zoarial hypostege, leucocytes, a mesenchyma- 

 tous plexus, and mandibular muscles. The only difference is in the abortion of the 

 polypide, which is reduced to the state of polypidian rudiment (= peculiar body, 

 ciliated organ). The mandibnlar muscles are powerful; there is an elevator bundle 

 (=abductor) of the mandible and an occlusor bundle ( = depressor, adductor, 

 retractor). The avicularian cavity is terminated by a beak. 



" The mandibles of the avicularia are symmetrical and have the closing muscles 

 attached by one or two long tendons. They all have a straight proximal edge, and 

 work from this straight base either against the calcareous bar, or, in case this is not 

 complete, then from two teeth." (Waters, 1913.) Avicularian glands may occur. 



The function of the avicularia is not yet known. It appears to be in relation 

 with the hydrostatic system. (Smittinidae, Metradoliwn obliquum, new species, 

 etc.) Many times the avicularia occur with the ovicell. They are always placed 

 in the vicinity of the apertura, and Canu considers them as being of service in 

 alimentation. Waters thought that they were organs of oxygenation. " The 

 polypide dies down from time to time, and a colony may be left with few or no 

 active polypides. It remains in vigor, through the avicnlarian organs retaining 

 vitality, and thus keeping the protoplasmic parenchyme in indirect communication 

 with the external surroundings. The avicularia have protoplasmic [mesenchyma- 



1 See D'Orbigny, Bryozoaires crfitacfis, pi. 719, figs. 7, 15 ; pi. 721, figs. 3, 7, 11 ; pi. 723, fig. 12 ; pi. 

 731, fig. 7, etc. The presence of zoarial tuberosities Is not necessarily a proof of hydrostatism, for these 

 same tuberosities can exist in zoaria composed of many superposed lamellae. 



