350 



COMPARATIVE ANATOMY 



sort of lymph, without blood corpuscles and without hemoglobin. It may 

 he assumed that the walls of these vessels are formed directly from the 

 surrounding mesenchyma and that the vessels therefore are evolved from 

 lacunar spaces. In the nematodes, a pseudocoelom provides an adequate 

 mechanism of circulation in animals which have no thick masses of tissue 

 to nourish. 



Most of the invertebrate phyla above the nematodes have composite 

 circulatory systems, partly lacunar and partly closed. Before Malpighi 

 (1661) discovered the capillary circulation in vertebrates and thereby 

 demonstrated in them a closed circulation, it was assumed that the verte- 

 brate circulatory system was likewise partly lacunar and partly closed. 

 That the lacunar system of invertebrates is comparable with the lymphatic 

 system of vertebrates has been more recently suggested. Such a sug- 

 gestion however is obviously based upon erroneous assumptions. The 

 facts accord better with the assumption that both blood-vascular and 

 lymphatic systems have had a common origin from the primitive lacunar 

 systems of invertebrates. While in the invertebrates the circulatory 

 system has remained partly open, in the vertebrates on the other hand the 

 circulatory system, both blood-vascular and lymphatic, has become 

 wholly closed. 



The vascular system of annelids is fundamentally like that of chordates. 

 In both groups occur two main longitudinal vascular trunks, one above 

 and one below the alimentary canal, connected with one another by aortic 

 arches around the pharynx. In the earthworm, there are five pairs of 

 aortic arches. It is true that the direction of flow of blood in annelids is 

 the reverse of that in vertebrates. But this difference in blood flow is 

 nullified if the dorsal and ventral sides of the worm are reversed. Reversal 

 of the annelid is also necessary in order to bring the central nervous sys- 

 tems of annelids and chordates into a similar position, dorsal to the aU- 

 mentary canal. It is therefore not surprising that proponents of the 

 annelid hypothesis of vertebrate ancestry have stressed the similarity of 

 the vascular systems of the two groups as a strong support of their views. 

 The absence of a heart in anneUds and its presence in vertebrates is not a 

 serious objection to this view, since the dorsal blood vessel of annelids is 

 contractile throughout its length and it may be reasonably assumed that 

 this contractile function is concentrated and localized in the vertebrate 

 heart. 



Such a diagram of the hypothetical primitive vertebrate blood-vascular 

 system as is shown in Fig. 291 is based, however, not on the assumption 

 of an anneUd ancestry of chordates but upon evidence from comparative 

 anatomy and embryology. While the circulation of blood in the ancestral 

 chordate was probably due, as in annelids, to the contractility of the walls 

 of the blood vessels, a contractile heart such as is found in all vertebrates 



