. Elementary text-book of zoology. Zoology. 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 o
. Elementary text-book of zoology. Zoology. 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 extensible 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 (fi
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
