< i \ss i BRYOZOA i'".!> 



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. 



Reproductive organs are developed in various parts of the cavity, the spermatozoa 

 usually in the 1> \\er, the ova in the upper portion. The ova may be developed in a 

 ipecial receptacle (marsupium) attached to the zocecium, or in an inflation of the 

 surface of the zoarium (yonocyst) ; in other cases, a modified zocecium (gonoecinm) is set 

 apart for reproductive functions. The general term ocecium or ovicell is applk-ulile t< 

 all of these structures. 



Many Bryozoans are provided with appendicular organs known as avicularia and 

 vibracula (Fig. 411). 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 ; but, 

 as a rule, especially among recent forms, they are pedunculate, 

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

 and Bicellaria, they resemble the head of a bird, consisting of a 

 lu Imet-shaped upper piece, with a formidable hooked beak, and 

 a mandible worked by powerful muscles. The jaws open and 

 close with 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 

 preservation, but their former presence on fossil specimens may 

 be generally determined by the slight pore-like excavations in 

 which they were lodged. The tubular spines, or acanthopores, B J g e ' n e l ' 

 which are of such common occurrence in Palaeozoic Bryozoans, portion of upper surface 

 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 Palaeozoic Bryozoans, which is curved to a 

 shorter radius and usually projects above the plane of the zocecial apertura Mesopores 

 are angular or irregular cells occupying interzocecial spaces in certain Palaeozoic 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'Orbign/s comprehensive system is largely artificial, and 

 although numerous modifications and improvements have been suggested by later 

 authors, a thorough revision has still to be undertaken. 



Lankester divides the class into two very unequal sub-classes 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 

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

 process, so that the tentacles form a discontinuous series. 



The Holobranchia are again divisible into two unequal groups, named by Nitsche 

 Ectoprocta and Entoprocta, according as the lophophore surrounds the mouth only, or 

 encloses both the oral and anal orifices. The first of these groups contains the bulk of 

 the known Bryozoa ; and, furthermore, the marine forms, and all genera capable of 

 preservation in the fossil state, are included in the single order Gymnolaemata. This 



