HIGH SCHOOL ZOOLOGY. 



79 



sucking rDontlis, which are armed with homy teeth but not 

 supported by jaws, to the bodies of other fish and prey upon 

 them. In genex-al shape they are eel-like : sevei-al species are 



kno^^^l, some of them marine, but the 

 commonest inland species is the silvery 

 lamprey, Petrorayzon aryenteus (Fig. 56). 

 Apart from the stiiicture of the mouth, 

 they are singular in the respiratory 

 organs, which have seven separate aper- 

 tures on each side, but only one opening 

 into the gullet. The marine genus 

 Myxine has similar habits, but different- 

 ly ari-anged gills. In all the forms the 

 skeleton is cartilaginous, and the noto- 

 chord foi-ms the bulk of the vertebi-al 

 column. 



31. A still further departure from the ordinary vertebrate 

 type is seen in the Lancelets, (Amphioxus or Brancliiostoma) 

 (Fig. 57) little fish-like creatures which burrow in the sand of 



Fig. 56.— Mouth of River 

 Lamprey. 



Petrorayzon argenteus. 



Fig 57 — Amphioxiis lanceolatus. 

 (After Glaus.) 



C, oral cirri; ch, notochord ; rm, spinal chord; ks, gills; ov, ovary; 1, liver: 

 N, kidney ; P, branchial pore ; A, anus. 



the sea coast. They lack the brain, skull, and heart of the verte- 

 brates, but the spinal cord and notochord are pre.sent, and the 

 anterior part of the alimentary canal is employed for respiration. 

 32. The same is true of the marine Tunicata, so called on ac- 

 count of the tunic containing cellulose, secreted by the skin 

 around the body They pass through a larval tadpole-like phase, 

 but afterwards lose the tail, and with it the notochord and over- 

 lying nervous cord, adopting for the most part a stationary Hfe, 

 dunng which many of them form colonies by budding. 



