72 



MOLLUSCOIDEA. 



also calcareous, rarely gelatinous exoskeleton, which arises from the 

 hardening of the cuticle around the individual zooids. Each zooid 

 (zocecium) (fig. 545) is accordingly surrounded by a very regular 

 and symmetrical case the ectocyst or cell ; through the opening of 

 which the anterior part of the soft body of the contained zooid with 

 its tentacular crown can be protruded. 



The form of the cells, and the manner in which they are connected 

 together, are very different in the different groups, and give, rise to 

 a great variety in the form of the colonies composed of them. The 

 cells are usually completely shut off from each other. With regard 



to their connection, they sometimes 

 project obliquely or at a right angle ; 

 sometimes they are spread out hori- 

 zontally on the same plane ; some- 

 times arranged in rows on a branched 

 axis. Their openings are usually 

 turned towards one side or towards 

 two opposite sides. The soft body 

 wall, or endocyst (fig 545, En} is 

 closely applied to the inner wall of 

 the ectocyst : it consists of an ex- 

 ternal layer of cells (matrix of the 

 ectocyst) and of a network of crossing 

 muscular fibres (the external fibres 

 are transversely, the internal longi- 

 tudinally arranged), which are 

 separated from the first layer by a 

 homogeneous membrane. On the 

 inner side of the muscular layer 

 there is, at least in the fresh-water 

 Bryozoa, a delicate layer of ciliated cells which line the body 

 cavity. At the opening of the cell the soft endocyst is invaginated 

 inwards, and passes thence on to the anterior and extrusible part of 

 the body, of which it forms the only investment. In most fresh- 

 water Bryozoa this reduplicature of the endocyst is always present 

 even when the zooid is protruded (fig. 545). The greater part of the 

 anterior region of the body, with its crown of tentacles, can, however, 

 always be protruded from the cell and retracted into it again by 

 special muscles traversing the body cavity (fig. 545). 



The disc on which the mouth is placed is known as the lophophore. 

 The Lophophore is either circular (Stelmatopoda), or it is drawn out 



SI 



FIG. 545. Plumatella repens (after 

 Allman). T, Tentacles ; L, lophophore ; 

 Oe, oesophagus; Mg, stomach ; A, anus ; 

 F, f uniculus ; St, statoblasts ; Ts, ten- 

 tacular sheath; Ek, ectocyst; En, 

 endocyst ; Gg, ganglion ; Pvm, parieto- 

 vaginal muscles ; Rm, retractor muscle. 



