390 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



with which they were formerly confounded under the name 

 of " Corallines." Like the Sertularians, they almost always 

 form compound aggregations, produced by repeated acts of 

 gemmation from the primitively single embryo, and have a 

 hard cuticular exoskeleton, which remains when the soft 

 parts decay. The compound organism thus formed is termed 



Fig. 112.— A portion of the polyzoarium of PlumateUa repens (after Allman). 1 



a Polyzoarium (Fig. 112), and each zooid which buds from 

 the common stock is a jPolypide. The outer, chitinous or 

 calcified, cuticular exoskeleton is termed the ectoeyst, and, as 

 the rest of the body of the polypide is contained in, or can 

 be retracted into, the hard case thus formed, it is commonly 

 termed a "cell." 



The proper ectoderm, with the parietal layer of the meso- 

 derm which lines and secretes this cell, is termed the endo- 

 cyst. The mouth is situated on a disk, termed the lopho- 

 phore, at the free end of the polypide ; and the margins of 

 the lophophore are produced into a number of richly ciliated 

 tentacula. At the oral aperture, the ectoderm passes into the 

 endodermal lining of the alimentary canal, which is almost 

 always divided into three portions, a long and wide pharynx, 

 a spacious stomach, and a narrow intestine. The latter is al- 

 ways bent up nearly parallel with the phaiynx, and termi- 

 nates in an anus situated beside the mouth. As the nervous 

 ganglion is placed between the mouth and the anus, the flex- 

 ure of the intestine is neural? and the ha?mal face of the 



1 " Monograph of the Fresh-water Polyzoa," 1856. 



2 In dealing with the morphological relations of the parts of Mollusks, it is 

 very necessary to employ a terminology which shall be independent of the or- 



