.. 



47 O POLYZOA cha 



which opens to the exterior by the orifice (0). The cavity is 

 bounded by a thin membrane termed the " tentacle-sheath " (ts), 

 and it is incompletely subdivided, near its upper end, by a 

 diaphragm (d), perforated by a circular hole through which the 

 tentacles can be protruded. The diaphragm bears the thin 

 folded collar characteristic of the Ctenostomata, the group to 

 which the species figured belongs (see p. 477). 



Fig. 238, B, shows the tentacles of Boiverbankia in their fully 

 expanded and partially expanded condition., Comparing this with 

 Fig. 234, it will be clear that when protrusion is taking place, 

 the tentacles are forced in a bundle, tips first, through the 

 diaphragm and next through the orifice of the zooecium, the 

 alimentary canal offering no resistance to this movement, owing 

 to the length of the oesophagus. A moment's consideration will 

 show that the bases of the tentacles, in passing through the 

 orifice, will carry with them that part of the flexible tentacle- 

 sheath to which they are attached ; and it will further be clear 

 that so much of the tentacle-sheath as is thus protruded will be 

 turned inside out. This process of " evagination " continues until 

 its further progress is stopped by the retractor-muscles (r.m), and 

 by the parieto-vaginal muscles (p.v), which pass from the interior 

 of the body- wall to the upper part of the tentacle-sheath. The 

 latter has now become the delicate layer which connects the 

 expanded tentacles with the zooecium; and the anus (Fig. 238, 

 C, a) opens directly to the exterior. Since the name " tentacle- 

 sheath " is thus descriptive of the condition of retraction only, the 

 term "kamptoderm" 1 has been suggested as an alternative name. 



The presence of a complete digestive canal and the ciliation 

 of the tentacles in Polyzoa are conspicuous differences between 

 these animals and the Hydroids, with some of which the Polyzoa 

 may have a marked external similarity. 



The outermost 2 layer of the body-wall is known as the 

 "ectocyst" (Fig. 234, e). This may be densely calcareous, in 

 which case the dried Polyzoon differs little in appearance from 

 the living animal with its tentacles retracted ; or it may be 

 partially calcified, or it may consist entirely of a flexible cuticle, 



1 Kraepelin, Abh. Ver. Hamburg, x. 1887, No. ix. p. 19; Ka/jL-rrreiv, to bend; 

 depfxa, skin. 



2 Parts of the ectocyst of some calcareous forms are covered by an external invest- 

 ment of cells, which give rise to secondary thickenings, ridges, and other growths. 



