98 



COMPARATIVE ANATOMY 



CHAP. 



1. Eetodermal supporting and protective organs. — These are 

 found in attached Cnidaria, and especially in those which form animal 

 stocks by asexual reproduction (incomplete fission and gemmation). 

 We can easily understand why such stocks, which in their natural 

 state imitate trees, bushes, grass, feathers, crusts, leaves, etc., need 

 special adaptations for holding the parts upright in the water, to 

 support and at the same time to protect them. We also see why 

 such supporting organs are of no use, or of very little use to 

 attached Cnidaria which do not form colonies, and why they are often 

 wanting, or only slightly developed in such forms. (Examples : Hydra, 

 the attached Scyphomechisce, and the Actinia among Corals.) 



The ectodermal supporting formations are simplest in the Hydroids. 

 Here the body epithelium generally secretes a chitinous cuticle (peri- 



FiG. V6.— Diagrammatic representation of the structure of a Stone Coral (MacLreporarlan), 

 after v. Koch. Only the lower aboral portion of the body is taken into consideration, fp, Foot- 

 plate ; ap, exotheca ; Tivp, theca ; ss, sklerosepta ; hs, sarcosepta. White parts = calcareous skeleton. 

 Streaked parts = ectoderm. Black parts = mesoderm. Dotted parts = endoderm. 



derm) which surrounds the body like a tube. This tube surrounds 

 either only the stem, the branches of the stock, and the stalks of the 

 individuals, or, further, it widens out round the individuals into cups 

 into which they can be withdrawn. In the division of the Hydro- 

 corallia the periderm becomes calcified, and forms a framework of 

 many tubes, arranged in a complicated manner, and reticulately bound 

 together. 



The ealeareous skeletons of the Stone-corals {HexacoralUa, Madre- 

 poraria), and the horny skeletons of the Alcyonidce (Octacffrallia) belong 

 to the order of skeletons secreted by the ectoderm. 



The origin of the calcareous skeleton of the Stone-corals (Fig. 76), 

 and its relation to the soft parts of the body, are as follows : — 



The young Coral, still devoid of skeleton, having attached itself 

 by the aboral end of its body, secretes from the ectoderm of its pedal 



