INVERTEBRATE ZOOLOGY. 47 



Cladus IV. Vertebrata. 



Sub-cladus I, Cyclostomata. 



This Stib-cladiis inchides two j^i'oups which may have the value of families. 

 For general anatomical details see the definition of the main group. 



Famil}^ I. Pefromyzontidae Nasal cavity a median blind-sack. Mouth 



(Lamprey eels.) without tentacles. They attach them- 



selves by their circular mouth to the 

 sides of fishes and stick their blood. 

 Fresh water and marine forms. 

 if Petromyzon. 



Family 2. Myxinidac Nasal sack provided with an inner pala- 



(Hag-fish.) tal opening. Mouth vsdth tentacles. 



Habits more parasitic than the former. 

 They push their waj^ even into the body- 

 cavity of other fish and consume their 

 viscera. All marine. -^ Myxine. 



Sub-cladus II. Gnathostomata. 



Division 1. ANAMNIA Amnion and other embryonic membranes 



fail. Breathe by gills during at least a 

 portion of their life. Mesonephros func- 

 tions as permanent kidney. 



Class I. Pisces Acjuatic Anamnia. with branchial respir- 

 ation. Paired limbs in the form of fins 

 (Ichthyopterygiiim). Integument gener- 

 ally produces scales, which contain bony 

 material produced by the corium. Head 

 with one auricle and one ventricle. 



Order 1. Selachii (Elasniobranchii). .Skeleton cartilaginous, scales placoid. 



Tail heterocercal. 5-7 separate gill open- 

 ings without oxierculum. -^ Squalas. 



Order 2. Ganoidei Skeleton more or less reinforced by bone. 



Scales overlaid with enamel and typically 

 rhomboid in form (ganoid). Tail hetero- 

 cercal. Gill-slits with operculum. 



