4 MORPHOLOGICAL TYPES 



It is customary to refer to the dorsal segmented regions of 

 the mesoderm as vertebral plate, and to the ventral unsegmented 

 portions as lateral plate. 



The gut is primitively straight, leading from a mouth at 

 the anterior end to an anus ; the latter is not at the extreme 

 posterior end of the animal but some distance in front of it. 

 Behind the anus is a well developed tail containing tissue 

 derived from all three germ-layers. The possession of such 

 a structure is one of the characteristics of the type as opposed 

 to most invertebrates. 



The blood flows in well-marked channels, and the direction 

 of flow is forwards ventrally and backwards dorsally, which is 

 the reverse of the invertebrate condition. The heart is 

 ventral. Blood is led from the intestine to the liver by a 

 hepatic portal vessel. A " portal " vessel is a vein which 

 differs from others in that it not only starts from capillaries, 

 but breaks up into capillaries again at the other end. The 

 hepatic portal vein therefore runs from the capillaries of the 

 intestine to those of the liver, and " carries " digested food- 

 products thither. Ordinary veins do not break up into 

 capillaries again, but connect with other veins and lead to the 

 heart. 



The nervous system is in the form of a hollow tube which 

 runs all the way down the dorsal side of the animal, and 

 contrasts sharply with the chief invertebrate type of two solid 

 ventral nerve-cords, swelling out into ganglia in each segment. 

 In vertebrates, the nerves are of two kinds, issuing from the 

 nerve- tube by dorsal or by ventral roots. 



The primitive respiratory system of vertebrates is equally 

 distinctive, and consists of a number (usually five or six) of 

 pairs of openings which lead from the front part of the gut to 

 the outside — the gill-slits with their contained gills. These 

 structures are among the most important, not only on account 

 of their distinctiveness, but also because of the modifications 

 which they undergo and the consequences which follow from 

 their possession in evolution. The higher vertebrates breathe 

 by means of lungs, which are sacs pushed out from the gut. 



Running down the back of the animal, above the gut and 



