46 



AN INTRODUCTION TO ZOOLOGY. 



tubes as fine as hairs); then these capillaries run 

 together again, joining again and again exactly as they 



Fig. 11.— Diploblastic animal with no separation between body cavity and 

 alimentary canal — transverse section of Hi/dra, magnified: JBc, ectoderm (epi- 

 blast) ; Ell, endoderm (hypoblast) j E, enteron or digestive cavity (see p. 40). 



u, Nuclei of cells ; i, interstitial cells; d, vacuoles of endoderm cells ; I, sup- 

 porting lamella (seep. 130). 



iAL 



Fig. 12.— Triploblastic animal with body-cavity (p. 40) and alimentary- 

 canal. Plan of transverse section of Earthworm (see p. 132) : b c, body-cavity 

 (shaded) surrounding oX. c, cavity of the alimentary canal (shaded) ; sfc, the skin, 

 represents the outer body- lay er, ectoderm or epiblast, and al, the lining of the ali- 

 mentary canal, represeats the inmost body-layer, endoderm or hypoblast ; 

 between these lie the structures comprised under the term mesoblast, including 

 w, muscular portion of the body-wall, and oo^, muscular portion of the wall of 

 the alimeiitaiy canal. The longitudinal nervous system, n, is ventral in posi- 

 tion. (The scale of the diagram is not larg"e enough tor cellular structure to 

 bo represented.) 



before branclied, to form veins, which carry the blood 

 back to the heart. 



