3O2 MANUAL OF ZOOLOGY. 



and we shall then have the actual condition presented by the 

 Polyzoa in their fully-developed state/' 



The vast majority of the Polyzoa are fixed, but this is not 

 universally the case. Thus the singular fresh-water Cristatella 

 is free and locomotive, creeping about by means of a flattened 

 discoid base, not unlike the foot of the Gasteropoda. 



The two fundamental structures of the " ccencecium " of a 

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

 dary investment are sometimes termed the " endoderm " and 

 " ectoderm ; " but as these terms are employed in describing the 

 Hydrozoa, 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 n are formed. In Cristatella, alone of the 

 Polyzoa, there is no ectocyst, and in Lophopus (fig. 124, 3) 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. 123, 

 3) and "vibracula." The avicularia, or "bird's head processes," 

 differ a good deal in shape, but consist essentially 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 Echinoder- 

 mata* 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 



* 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 pedicellarice of the Echinodermata, continue 

 their movements long after the death of the animal. 



