MANUAL OF NATUKAL HISTOKY. Ill 



I. SUB-ORDER. KOUND-MOUTHED SuCTORIAL-FlSHES 



(Marsipobranchii, seu Cyclostomi). 



Gills fixed, bursiform, inoperculate, receiving re- 

 spiratory streams by apertures usually numerous 

 and lateral, distinct from the mouth ; a heart. 



1. Family. — Lampreys (Petromyzontidse). Body 



elongated ; eyes small or none ; mouth cir- 

 cular; teeth distinct; branchial openings 

 mostly seven ; nasal aperture closed ; palate 

 entirely covered with skin. 



2. Family. — Glutinous- Hags (Myxinidse). Body 



cylindrical ; eyes none ; branchial apertures 

 two ; a single tooth on upper part of maxil- 

 lary ring ; palate perforated. 



II. SUB-ORDER. — Fkinged-mouthed Suctorial-Fishes 

 (Pharyngobranchii, seu Cirrhostomi). 



Gills free, pharyngeal, inoperculate ; no heart. 



3. Family. — Lancelets (Amphioxidse). Body com- 



pressed, hyaline ; mouth narrow, elongated, 

 the margins furnished with slender filaments. 



MOLLUSKS. 



The unsymmetrical, soft, and fleshy bodies of the 

 animals forming the Sub-Kingdom of the Mollusks 

 have no internal bony skeleton to protect the ner- 

 vous centres, nor are they covered with a horny, 

 jointed skin like the tribes of Annulosa. Nature, 

 however, not to leave them altogether destitute in 

 this respect, has invested them with a muscular 



