olfocfory chamber 

 nasal valve 

 nasohypophyseal opening 

 anterior and posterio 

 tectal cartilages 



subhypophyseol sac 



spinal cord venous sinuses 



esophageal pha 

 'respiratory pharynx 

 afferent branchial openings 



liver 

 pericardial cortilage 



bile duct 



velar cartilage 



opens to left side of typhlosole 



velar tentac 



B 



velar valve first afferent branchial opening Q islet tissue 



Figure 9-26. Sagittal section through the head region of a lamprey. 



9-28). The gall bladder lies between these lobes, and its 

 short duct extends forward to enter the intestine slightly to 

 the left of the midline and well behind the union of the 

 pharynx and gut. 



At the point where the bile duct enters the gut, there is a 

 swelling marking the mass of islet tissue of the pancreas. 



There is no spleen in either Myxine or Peiromyzon, although 

 the latter has much lymphoid tissue in the typhlosole of the 

 larva. The myxinoid has myeloid tissue in the submucosa of 

 the gut, while the adult lamprey, on the way to spawn, has 

 some cell production going on in the blood itself and in the 

 kidney tissue. 



The development of the my.xinoid is not well known. The 



telolecithal egg develops much as in the shark or teleost 

 ( Figure 7-14). Development is direct, without metamorphosis. 



General observations on fishes 



The abdominal viscera of fishes show the same basic pat- 

 tern as the tetrapod both in terms of adult parts and in the 

 developmental stages of these parts. The diffuse pancreas of 

 the actinopterygian and the internal pancreas of the dipnoan 

 or cyclostome raise the question of the primitive condition. 

 The fact that the shark, or holocephalan, and Latimeria have 

 a well-developed pancreas suggests that this was the original 



tentacles 



palatine commissure 

 nasal tube cartilages 



nasal capsule olfactory fold in olfactory chamber frenulum of velu 



nosohypophysea 

 opening 



subnasal cartilage 



ethmoid tooth 



anterior lingual cartilage 



dentigerous cartilage'' / protrbctor muscle of dental plate / \ retractor muscle of dental plate 



medial lingual cartilage ligament of retractor muscle posterior lingual cartilage 



Figure 9-27. Sagittal section through the head region of Myx'me. 



278 



THE VISCERA 



