126 ZOOLOGY SECT. 



and is hence known as the spiral valve. There is no continuous 

 mesentery, but a number of narrow supporting bands. 



The liver (Fig. 806, Ir.) is a large bilobed 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 are a few follicles on the surface of the liver, 

 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. 806, 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 connec- 

 tion 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, with numerous gill-lamellae developed on the inner 

 surfaces, and are separated from one another by wide interbranchial 

 septa. In the larva an additional 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 aortce. 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 vein (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 left precaval disappears in the adult, so that 

 the jugulars and cardinals of both sides open into the right precaval. 

 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. 807, 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, 

 so that when the membranes of the brain are removed, an aperture 

 is left which is covered in the entire organ by a vascular thickening 

 of the pia, or choroid plexus (ch. pi. #). On the dorsal border of 



