. Elements of Comparative Anatomy. DERMAL SKELETON OF ECHINODERMA. 201 d d' c). The whole body is covered with cilia at a period before that when they are arranged on the ridge-like projections from the ciliated band; but this general ciliation is only found during the most indifferent condition of the larva. The cilia are retained even later on, on many parts of the soft dermal layer which invests the calcareous skeleton: as, for example, in the ciliated tracts, which reach to the mouth in the Spatangidte (semitaa). On other parts, such as the dermal branchite (cf. supra), ciliation appears t
. Elements of Comparative Anatomy. DERMAL SKELETON OF ECHINODERMA. 201 d d' c). The whole body is covered with cilia at a period before that when they are arranged on the ridge-like projections from the ciliated band; but this general ciliation is only found during the most indifferent condition of the larva. The cilia are retained even later on, on many parts of the soft dermal layer which invests the calcareous skeleton: as, for example, in the ciliated tracts, which reach to the mouth in the Spatangidte (semitaa). On other parts, such as the dermal branchite (cf. supra), ciliation appears to be correlated with the respiratory function of the integument, in which the ambulacral feet may also have a share. The extent to which the integument is calcified varies gi'eatly. Sometimes the calcareous particles are united with one another into larger pieces, and form plates which are movably or immovably connected together : this arrangement either extends over the whole body, or is confined to definite tracts of its surface. In other cases the calcareous particles are scattered, and allow of great variations in the form of the body. In this case a large number of the characteristic features of the Echinoderm disappear from other parts of its organisation; so that the disappearance of a calcified integu- ment is a departure from the type, and the general phEenomenon of a scanty deposit of calcareous matters is not to be regarded as an early, but as a final stage in the series of forms. Calcification converts the integument into an organ of support for the body, or dermal skeleton ; in many cases this sends out pro- cesses into the interior of the body. These give rise to calcified struc- tures, which form an internal skele- ton and combine with the external. The whole thickness of the peri- some is not affected by the process of calcification. A thin non-calci- fied layer of tissue is always found on the inner, as well as on the outer surface; on the latter, however, this
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Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
