236 MANUAL OF ZOOLOGY. 



Polyzoon, viz. the immediately investing sac, and its secon- 

 dary investment, are sometimes termed the ' eridoderm ' and 

 ' ectoderm,' but as these terms are employed in describing the 

 Hydfozoa, it is better to make use of the terms * endocyst ' and 

 * ectocyst,' proposed by Dr. Allman. 



The ' ectocyst,' or external investment of the coenoecium, 

 is usually a brown, pergamentaceous, probably chitinous, but 

 often highly calcareous, membrane ; and it is by the ectocyst 

 that the ' cells ' are formed. In Cristatella, alone of the Polyzoa, 

 there is no ectocyst, and in Lopliopus the ectocyst is gelatinous 

 in its consistence. In many cases the ectocyst is provided 

 with singular appendages, supposed to be weapons of offence 

 and defence, termed ' avicularia ' (fig. 72, 3) and ' vibracula.' 

 The avicularia differ a good deal in shape, but consist es- 

 sentially of ' a movable mandible and a cup furnished with a 

 horny beak, with which the point of the mandible is capable 

 of being brought into apposition.' (Busk.) In shape the 

 avicularia often closely resemble the head of a bird, and they 

 are in many respects comparable with the ' pedicellariae ' of 

 the Ecliinodermata* In the 'vibracula,' the place of the 

 mandible of the avicularium is taken by a bristle, or seta, 

 which is capable of extensive movement. 



The endocyst is always soft, contractile, and membranous. 

 It lines the interior of the cells formed by the ectocyst, and is 

 reflected backwards at the mouth of the cell, so as to be inva- 

 ginated, or inverted into itself ; and it finally terminates by 

 being attached to the base of the circlet of tentacles. This 

 invagination of the endocyst is more or less permanently pre- 

 sent in all the fresh- water Polyzoa. A portion of the inner 

 surface of the endocyst, if not the whole, is furnished with 

 vibratile cilia. 



The mouth of each polypide is surrounded by a crown of 

 tubular, non-retractile tentacles, which have their sides ciliated, 

 and are arranged sometimes in a circle, and sometimes in a 

 crescent. The tentacles are borne upon a kind of disc, or stage, 

 which is termed by Professor Allman the ' lophophore.' In the 

 majority of Polyzoa including probably almost all the marine 

 Species the lophophore is circular, but in most of the fresh- 

 water forms it has its neural side extended into two long arms, 

 so that the entire lophophore becomes crescentic or ' horse- 

 shoe-shaped ' (fig. 73) ; hence this section is sometimes collec- 



* There is great reason, however, as shown "by Huxley, to regard the 

 avicularia, not as mere appendages or organs of any kind, but as peculiarly 

 modified zooids, having many singular points of affinity with the Brachio- 

 poda. The avicularia, like the pedicellarifB of the Echinodcrmata, continue 

 their movements long after the death of the animal. 



