GENERAL ZOOLOGY 



which apparently function in directing the flow of blood through the body 

 spaces. It will be recalled that in these tracheate forms the blood has 

 no oxygen-carrying capacity, this function being performed by the branching 

 tracheal tubules. 



Examples of well-developed and representative circulatory systems among 

 the invertebrates are thus found in the Annelida, the Mollusca, and the 

 Arthropoda; the fundamental functional requirements are met in these groups 

 by related though dissimilar structural specializations. The effective cir- 

 culatory mechanism of most echinoderms is unique, involving the coelomic 

 fluid which is kept in motion by the flagellated peritoneum. Among echino- 

 derms only holothurians and echinoids have a blood-vascular system, the 

 haemal system, retaining a residual circulatory function. In other phyla of 

 invertebrates circulatory systems are ab.sent or but slightly developed. Except 

 for certain invertebrates, such as the tracheate arthropods, general 

 functions of the circulatory system are similar wherever such a system 

 exists, whether in vertebrates or in invertebrates. The circulating fluid 

 brings nutrients, o.xygen, and in many instances hormones, to the tissues and 

 carries away for disposal the gaseous and nitrogenous waste products of 

 metabolism. Various circulatory mechanisms are shown .schematically in 

 Figures 17.2 and 17.3. 



Respiratory Systems. Typical animals must continuously exchange 

 oxvgen and carbon dioxide with their environments. Special organs to serve 

 this function are not commonh* found among invertebrates in which gas 

 exchange can be carried out over a large part of the body surface, as in the- 

 hydra and the earthworm. Generally speaking, development of such special 

 structures occurs concomitantlv with increase in size and thickness of the 

 body. This generalization follows the well-known fact that large bodies 

 have much less surface area per unit of volume than smaller bodies. The 

 flattened, leaf-like body form in the relatively large platyhelminth worms 

 may be interpreted as an adaptation that compensates for the lack of special 

 respiratory organs by decreasing the thickness of the body. Among inverte- 

 brates that have developed organs for respiration, the apparently diverse 

 mechanisms can be classified into a few general types, schematically repre- 

 sented in Figure 17.4. These structures are without exception related to 

 the environmental situation in which the animal lives, although they are 

 often conditioned by the ancestral history of the animal. Primarily aquatic 

 species are commonly provided with blood gills, which are outgrowths of 

 the surface of the body with a particularly copious blood supply. These may 

 be external gills, as in the branchial filaments and parapodia of polychaete 

 annelids (pp. 398-402); or, as in pelecypod mollusks and the larger, cru.s- 

 tacea, they may be enclosed within a cavitv opening at the surface. Many 

 echinoderms have dermal branchiae or other thin-walled outpocketings 

 of the body wall through which respiratory exchange occurs between coe- 

 lomic fluid and external sea water. Holothurians, again, are unusual in 

 their pos.session of the ''water lungs," which are functionally analogous 



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