CHORDATE ANATOMY 



Nearly 50,000 species are known. 



Four sub-phyla are included in the phylum — Hemichorda, Urochorda, 

 Cephalochorda, and Vertebrata. 



Sub-Phylum Hemichorda (Enteropneusta) 



The hemichordates or Enteropneusta hold a somewhat uncertain posi- 

 tion in the animal kingdom. Morphologists are by no means agreed that 

 their closest affinities are with the chordates. Some associate them with 



PENING OF PERIBRANCH.CAV. 

 /ANUS 



DORS. NERVE CX«D /NOTOCHORD 



IBmnniiiiimiiiiiiiiiiiiiiiiimimrniiiiiiiiiniiMMiiririiiiiTiiiflfiininm 



A. LARVAL UROCHORDATE 



GILL SLfTS' 



/DORSAL NERVE 



ANUS 

 POST OPENING OF 

 PERIBRANCHIAL CAVITY 



B CEPHALOCHORDATE 



.DORS. NERVE CORD 



KCILL SUT5 'HEART 



LIVER 



*> INTESTINE ANUSi 



C. VERTEBRATE (CYCLDSTOME) 

 Fig. 2. — Diagrams of A, larval urochordate, B, cephalochordate (Amphioxus), 

 and C, vertebrate (Petromyzon), illustrating the fundamental characteristics of 

 chordates; redrawn after Hesse- Doflein. 



the annelids, while the resemblance of their larval stage to that of echino- 

 derms leads others to place them near that group. Their inclusion among 

 the chordates rests on their possession of pharyngeal gill-sUts, enteric 

 coelomic pouches, a notochord-Uke diverticulum of the fore-gut in the pre- 

 oral lobe, and upon the relations of the blood-vessels and nerves. Seg- 

 mental excretory organs are, however, absent. 



There are possibly 50 species. 



BALANOGLOSSUS, the best-known genus, may be taken as representa- 

 tive. The body of Balanoglossus is worm-Hke, and is divided into five 

 regions, proboscis, collar, gill region, "liver" region, and intestinal region. 



