somites were without a coelomic space. The tail of the 

 primitive vertebrate is a new structure and does not corre- 

 spond to the stalk of the hemichordate. 



The notochord served as the axial support for this elongate 

 organism and was supplemented by a sheath of fibrous con- 

 nective tissue which later gave way to cartilage and bone. 

 The notochord extended forward only to the mid part of the 

 brain. The central nervous system lay above the notochord 

 and consisted of a five- (or six-) parted brain associated with 

 well-differentiated sensory organs (nose, eye, ear) and a 

 spinal cord extending most of the length of the body. Spinal 

 and cranial nerves extended out to each of the myotomes. 

 Each nerve had a dorsal and a ventral root, and the dorsal 

 root contained a sensory ganglion. There was a partly sepa- 

 rated autonomic system. Some of the ganglia were associated 

 with the viscera, and there were nerve nets within the gut 

 wall. The motor fibers of the nervous system were generally 

 myelinated. 



The pharynx had an evaginated thyroid gland that prob- 

 ably retained connection by a duct. The pharyngeal openings 

 were simple slits and few in number. Since new slits were 

 added or subtracted posteriorly, the number was variable. 

 The pharyngeal slits were independent of the myotomes, i.e. 

 not segmental. Each slit had gill filaments associated with 

 it. The mouth was separated from the pharynx by a velum, 

 and in it arose a midline Rathke's pouch and bilateral nasal 

 capsules. 



The gut was simple, having an expanded stomach and 

 liver and a pancreatic diverticula, which may not have been 

 well marked. The posterior part of the intestine contained 

 a spiral valve. The gut lay in a large bilateral coelom. 



Coelomic spaces also occurred in the head and in the bran- 

 chial arches. 



The circulatory system consisted of a ventral vessel, in 

 which the blood passed forward, and a dorsal vessel, close 

 below the notochord, in which it flowed posteriorly. There 

 was a contractile heart, enclosed in a pericardium, which 

 pumped the blood forward and upward through the bran- 

 chial arches. Anterior and posterior cardinal veins along with 

 jugulars and abdominal vessels returned the blood to the 

 heart. An hepatic portal vein drained the gut into the liver, 

 where it entered a sinusoid reticulum drained into the heart 

 by hepatic veins. The kidney was also associated with some 

 sort of portal system. Erythrocytes containing hemoglobin 

 were in the blood. 



Kidneys were present in the anterior part of the body 

 cavity. They were formed of nephric tubules opening into 

 the pericardial space or the coelom at one end and into a 

 common drainage duct at the other. These tubules lay 

 within a blood sinus. The reproductive organs were masses 

 of cells that ruptured into the coelom. The coelomic spaces 

 opened to the exterior by abdominal pores near the cloaca 

 (the common opening of digestive and excretory systems). 



Much more could be hypothesized but the general picture 

 has been presented. The origin and development of the ver- 

 tebrate from the invertebrate host can now be imagined 

 without resort to impossible morphological saltations. A 

 fuller appreciation of this transition, however, is dependent 

 upon a fairly detailed knowledge of the vertebrates. The 

 evolutionary radiation of vertebrates should now be investi- 

 gated in terms of the morphology of living forms and the 

 historical record. 



THE PROTOCHORDATES • 17 



