122 ZOOLOGY SECT, 



The liver (Fig. 752, lr.) is a large one-lobed organ, and is peculiar 

 from the fact that there is neither gall-bladder nor bile-duct in the 

 adult, except as an individual variation, although both are present 

 in the larva. There is a small gland opening into the intestine 

 which may represent a pancreas : the spleen is absent. Paired 

 glands imbedded in the muscles of the head, and opening into the 

 mouth, are known as " salivary glands." 



Respiratory Organs.- -The Lampreys differ from all other 

 Vertebrata in the fact that the gills do not open directly into the 

 enteric canal in the adult, but into a respiratory tube (Fig. 752, r.t.) 

 lying below the gullet. This is a wide tube opening in front into 

 the buccal cavity, and ending blindly a short distance in front of 

 the heart : in the larva it communicates behind with the intestine, 

 and is, in fact, the pharynx, the gullet of the adult being not yet 

 developed ; but at the time of metamorphosis it loses its con- 

 nection with the intestine, and the gullet is developed as a forward 

 extension of the latter an entirely new formation. The respiratory 

 organs are typical gill-pouches (br. 5) : they have the form of 

 biconvex lenses, and are separated from one another by wide inter- 

 branchial septa. In the larva an eighth cleft has been found in 

 front of the first of the adult series. 



Circulatory System. --The auricle (au.) lies to the left of the 

 ventricle (v.) and receives blood from a small sinus venosus (s. v.\ 

 There is no conus arteriosus, but the proximal end of the ventral 

 aorta presents a slight dilatation or bulbus aorta:. Both afferent 

 and efferent branchial arteries supply each the posterior hemi- 

 branch of one gill-pouch and the anterior hemibranch of the next : 

 they are thus related to the gills, not to the gill-pouches. In 

 addition to the paired jugulars (ju.) there is a median ventral 

 inferior jugular win (i. ju.} returning the blood from the lower 

 parts of the head. There is no renal-portal system, the two 

 branches of the caudal vein being continued directly into the 

 cardinals (cd.). The red blood-corpuscles are circular nucleated 

 discs. There is a large system of lymphatic sinuses. 



Nervous System. --In the brain the small size of the cerebellum 

 (Fig. 753, crb.) is remarkable : it is a mere transverse band roofing 

 over the anterior end of the metaccele. The optic lobes (opt. I. ) 

 are very imperfectly differentiated, 'and the central region of the 

 roof of the mid-brain is formed merely of a layer of epithelium, 

 giving rise to an aperture (ell. pi. 2) when the membranes of the 

 brain are removed, but covered in the entire organ by a vascular 

 thickening of the pia or choroid plexus. On the dorsal surface of 

 the diencephalon are two masses of nervous matter, the ganglia 

 kabenula 1 , the right (r. (jn. hi.) much larger than the left (/. gn. hi.) : 

 they are connected with the pineal apparatus. Below the dien- 

 cephalon is a small flattened pituitary body (Fig. 754, pty. I.). In 

 front of the diencephalon are paired bean-like masses, each con- 



