1 86 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



however, the important difference in the function of 

 the contained fluid in the lower as compared with the 

 higher Nemertinea. In the former haemoglobin in 

 distributed in the nerve tissue, but is absent from the 

 blood, so that that fluid has only nutrient functions 

 in the Schizonemertini, and not both nutrient and 

 respiratory duties as in the higher Hoplonemertini. 

 There are three chief longitudinal vessels, two lateral, 

 which are connected with one another at the anterior 

 end of the body, and one median, which is connected 

 with the two lateral a little behind the region of 

 the mouth. All these are contractile, but they are 

 of the same calibre throughout, or, in other words, 

 there is no special portion which is enlarged to act 

 as a pumping organ or heart. 



When we pass to those higher forms of Worms 

 in which metarneres are developed, transverse branches 

 or lateral vessels unite the median with a now 

 ventrally placed trunk, and some of these lateral 

 vessels become contractile (so-called hearts of 

 Ssenuris and others). 



The dorsal vessel (d) of such forms as, for example, 

 the earthworm, is retained in the crayfish as the an- 

 terior (Fig. 81; aa')and posterior aortse (pa); the trans- 

 verse vessels are indicated by the short arteries at and 

 hp, which supply the anterior regions of the body and 

 the viscera; one transverse vessel is still complete, and 

 forms the descending sternal artery (st.a) which 

 opens into the backwardly and forwardly directed 

 abdominal artery (si.a ; iaa) the representative of the 

 ventral vessel of the earthworm. In the Anodon (Fig. 

 82 c) the spaces or sinuses are much more developed, 

 and no indications of a ventral vessel are now to be 

 seen ; the dorsal is, however, shown by the heart (H) 

 with its anterior and posterior aortse (aa',pa) ; while 

 the terminal parts of the transverse vessels become 

 enlarged to form the auricles of the heart or ventricle (a). 



