CLASS I BRYOZOA 259 
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 lower, the ova in the upper portion. The ova may be developed in a 
special receptacle (marsupiwm) attached to the zocecium, or in an inflation of the 
surface of the zoarium (gonocyst) ; in other cases, a modified zocecium (gonoectum) is set 
apart for reproductive functions. The general term owetwm or ovicell is applicable to 
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 
helmet-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, BE cee Rie 
which are of such common occurrence in Palaeozoic Bryozoans, portion of upper surface 
C Do. 6o a showing a vibraculumand 
were, in part at least, probably the supports of similar structures. oyicell (after Busk). 
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 aperture. 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. 
Classtfication.—The classification of the Bryozoans remains as yet in an unsatis- 
factory condition. D’Orbigny’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 
Rhabdoplewra, 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 Bryozow ; and, furthermore, the marine forms, and all genera capable of 
preservation in the fossil state, are included in the single order Gymnolaemata. This 

Fic. 411. 
