GENERAL ZOOLOGY 



The rod-like bodies related to the locomotor apparatus in some flagellates, 

 formed within the cell, seem to function also as supporting skeletal structures. 



The vast majority of sponges produce some kind of supporting framework. 

 In calcareous and siliceous sponges, this mav consist of individual spicules of 

 calcium carbonate or of glass, or these spicules may be fused into a continuous 

 latticework. The Keratosa produce skeletons consisting of interlaced fibers of 

 the proteinaceous substance termed spongin. All these skeletal elements are 

 synthesized by individual mesenchyme cells called scleroblasts and first appear 

 as intracellular crystals or fibers (see Fig. 9.9, p. 277). In the finished con- 

 dition they may be enclosed within the mass of the sponge or lie at the surface, 

 so that a distinction between endo- and exoskeletons in the Porifera is 

 impossible. 



Among the simplest eumetazoans, which, unlike sponges, have evolved true 

 tissues, supporting structures are widespread and varied. Coelenterates like 

 Hydra and jellyfishes possess no skeletons, the function of support being 

 served by the mesolamella or the turgid mesogloea. Colonial hvdrozoans, 

 with the exception of siphonophores, commonly form tubular exoskeletons 

 known as the perisarc, as in Obeha. Anthozoans either have no skeletons, as 

 in Metridium and other sea anemones, or produce protective and supporting 

 structures varying from the calcareous, epidermal secretions of corals to the 

 organic skeletons of sea fans and similar forms. Inasmuch as these latter 

 skeletal elements lie deeply embedded in the body and seem to be of meso- 

 dermal origin, they may be considered endoskeletal. 



Among Platyhelminthes true exoskeletons are lacking, although protective 

 cuticular coverings are common in parasitic forms. It should be noted, how- 

 ever, that these organic covering layers are not homologous with the 

 epidermally produced cuticles and exoskeletons of higher forms. In adult 

 trematodes and cestodes the epidermis is lacking, and the cuticle is secreted 

 by underlying mesenchymal cells. 



A common pattern of skeleton formation appears throughout the pseudo- 

 coelomate and schizocoelous eucoelomate groups. The basic "skeleton" is a 

 cuticular covering, organic in nature, secreted by epidermal cells. This type 

 of protective covering is typical of all Aschelminthes and of annelid worms. 

 The skeletons of brachiopods, ectoprocts, and mollusks appear to be modifica- 

 tions of this basic cuticle, commonly made more effective by the deposition of 

 layers of calcium carbonate within and beneath the organic material. It will 

 be recalled that the shells of bivalves, for example, are covered by an 

 organic periostracum. Some cephalopod mollusks present interesting advances 

 correlated with their more active mode of life, in which a bulky calcareous 

 shell would be a handicap. The squid retains a remnant of the ancestral exo- 

 skeleton, an organic, axial supporting structure called the "pen," which has 

 become so surrounded by the tissues of the mantle as to resemble an endo- 

 skeleton. This animal has developed true endoskeletal elements, however, in 

 the head region; there is an internal cartilaginous structure which serves to 

 protect the brain and eyes and to anchor the muscles of the jaws and arms. 



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