BRITISH MARINE POLYZOA. 81 
opening inwards into the cell cavity. They are covered with vibra- 
tile cilia, ranged in a single line along each side, and serving to 
produce ciliary currents. These cilia project from cells with large 
nuclei, which line each side of the tentacle. The tentacles vary in 
number from 8 to 80. These tentacles perform functions which 
are of great importance. They are really respiratory organs or gills, 
and it is the possession of this feature which really assigns the 
polyzoa to their place so high in the animal scale as the Mollusca. 
They are also tactile organs, and receive all the impressions from 
without which the zooid is capable of receiving. They are capable 
of very active movements, and are provided with an admirable and 
effective service of muscles. <A pair of these muscles in the form 
of a collection of fibres being found in each tentacle. The tentac- 
ular sheath is a membranous expansion of the anterior part of the 
zocecium, and is attached by its end to the base of the tentacles. 
The movements of the polypide are limited to the expansion and 
retraction of this sheath and the consequent extension and retrac- 
tion of the tentacles. The cavity of the cell is sealed. By the 
“orifice” of the zocecium is meant, not an opening in the ordinary 
sense of the term, but that opening in the structure of the zoceclum 
through which the tentacular corona and sheath may be protruded. 
The orifice is a very important element in classification. 
II. The alimentary canal has three well-marked divisions. The 
cesophagus, the stomach, and the intestines. The mouth opens 
into the cesophagus, which is thickly clothed with cilia, which assist 
in sending the food downwards into the stomach. ‘The upper part 
of the cesophagus is often a very distinct funnel-shaped pharynx, 
and is then a very wide chamber with well-adapted muscular walls. 
The surface of the pharynx is dotted over with minute spots. 
The cesophagus varies much in length; though generally it is a 
simple tube, leading directly into the stomach. In the course of 
the tube is placed a valve (the caraza), which is a conical perforated 
projection with the free end downwards. ‘This marks the entrance 
to the stomach, and is so placed as to prevent the return of any 
food once received into the stomach. 
The cesophagus is furnished with transverse muscular striz, 
which by contracting the tube aid in forcing down the food. The 
stomach is contracted by similar muscular means. The stomach 
is a sac or bag, wide above and more or less pointed below. It has 
thick walls of a rich yellowish brown colour, due to the pressure in 
its lining membrane of numerous glands, which perform the func- 
tions of a liver and secrete a brown fluid, probably of biliary action. 
The intestine opens out of the stomach at its upper end near the 
cesophagus. The entrance to the intestine is guarded by the 
pyloric valve performing similar functions to the cordia. This 
valve is sometimes placed at the summit of a conical chamber, 
