316 MOLLUSCOIDEA phylum v 



out delicate nerve filaments to the tentacles and oesophagus, lies between the mouth 

 and anus. The upper or anterior part of the sac is generally flexible and admits of 

 being invaginated by the action of numerous, longitudinal and transverse muscles, 

 which traverse the fluid-filled visceral cavity. 



Eej)roductive organs are developed in various parts of the cavity, the spermatozoa 

 usually in the lower, the ova in the upper portion. The ova may be developed in a 

 special receptacle {marsiqnum) attached to the zocecium, or in an inflation of the 

 surface of the zoarium (gonocyst) ; in other cases, a modified zocecium {gonoecium) is set 

 apart for reproductive functions. The general term ocecium or ovicell is applicable to 

 all of these structures. 



Many Bryozoans are provided with ajipendiciilar organs known as avicularia and 

 vibracula (Fig. 436). Their functions are somewhat doubtful, some authors regarding 

 them as food-procuring agents, and others as organs of defence. 

 The avicularia may be immovably attached to the zocecium ; 

 1jut, as a rule, especially among Recent forms, they are peduncu- 

 late, and capable of considerable swaying motion. Often, as in 

 Bugula and Bicellaria, they resemble the head of a bird, con- 

 sisting of a helmet-shaped upper piece, with a formidable hooked 

 beak, and a mandible worked by powerful muscles. The jaws 

 open and close wdth a perpetual snapping motion, and small 

 organisms or other foreign particles happening in their way are 

 seized and held with a tenacious grasp. The vibracula are 

 flexible, bristle-like appendages, generally set in the excavated 

 summit of a knob-like elevation, or on a blunt spine. 



The avicularia and vibracula are themselves incapable of 



Fig. 430. preservation, but their former presence on fossil specimens may 



Selenaria maculata be generally determined by the slight pore-like excavations in 



porMon^oTu°ppef slfrlce which they were lodged. The tubular spines, or acanthopores, 



showing a vibracuium which are of sucli common occurrence in Paleozoic Bryozoans, 



were, in part at least, probably the supports of similar structures. 



The term lunarium is applied to a more or less thickened portion of the posterior 



wall in many Paleozoic Bryozoans, which is curved to a shorter radius and iL?ually 



projects above the plane of the zooecial aperture. Mesopores are angular or irregular 



cells occupying interzooecial spaces in certain Paleozoic genera. 



Most Bryozoans are attached, either by the greater part of their surface, or only 

 basally, to extraneous objects ; or they are moored to the bottom by root-like appen- 

 dages. In many forms the zoarium is regularly jointed. The majority of genera 

 inhabit the sea, and occur in all zones and at all depths ; only a few genera live in 

 fresh water. The animals subsist chiefly on Diatoms, Infusorians and larvae. 



Classification. — The classification of the Bryozoans remains as yet in an unsatis- 

 factory condition. D'OrViigny's comprehensive system is largely artificial, and 

 although numerous modifications and improvements have been suggested by later 

 authors, further revision has still to l)e undertaken. 



Lankester divides the class into two very unequal subclasses as follows : (1) 

 Holobranchia, in which the lophophore or row of tentacles is unbroken, and either 

 circular or horse-shoe shaped ; and (2) Pterobranchia, containing the single genus 

 EJtabdopleui-a, which has the lophophore produced on either side into a plume-like 

 process, so that the tentacles form a discontinuous series. 



A more modern system is to regard Bryozoans as a primary group or phylum, 

 which is divided into two unequal classes, named l)y Nitsche, Ectoprorfa and Ento- 

 procta, according as the lopho])hore surrounds the mouth only, or encloses both the 

 oral and anal orifices. The first of these classes contains the bulk of the known 

 Bryozoa. Furthermore, the marine forms, and practically all genera capable of pre- 

 servation in the fossil state, are included in the subclass Gymnolaemata Allman. 



