228 GENERAL ZOOLOGY 



which it is well-developed and functional. The Larvacea 

 (a) therefore .represent the most primitive types. Most of 

 the common sea-squirts have, like barnacles (Fig. 32), 

 undergone retrogressive metamorphosis, and as adults are 

 adapted to a sessile existence. 



Subphylurn. 3. Cephalochorda. The lancelets (Fig. 90, e) 

 are small fish-like animals which bury themselves tail first 

 in sandy ocean beaches, below low-tide mark. Amphioxus 

 is a well-known representative which has been much dis- 

 cussed by evolutionists as a possible ancestor of vertebrates. 

 It has a well-developed chorda and numerous paired gill 

 slits opening from the pharynx to the exterior. The body 

 wall is divided into a large number of muscle segments, or 

 myomeres. Along the whole dorsal margin there is a nar- 

 row fin which is continuous posteriorly with a broader 

 caudal fin. On the ventral side of the body, a short medial 

 fold (ventral fin) reaches forward from the caudal fin, and 

 in front of this two lateral (metapleural) folds extend to 

 the region of the mouth. The median fins and the lateral 

 folds are continuous, but have the same arrangement as 

 the separate fins of true fishes. The metapleural folds 

 have, therefore, been considered to be the forerunners of 

 the paired limbs of the vertebrates. The cephalochord- 

 ates excel Enteropneusta and Tunicata in their greater 

 cephalization and show closer affinities with the vertebrates. 



Subphylum 4. Vertebrata. This group includes the eel- 

 like cyclostomes, sharks and skates, salamanders, frogs, 

 reptiles, birds, and mammals. Most vertebrates have two 

 characteristics which usually serve to separate them readily 

 from other chordates: (1) vertebrae replace the chorda, 

 which is usually present as such only during embryonic 

 development; (2) two pairs of lateral appendages, or limbs, 

 are present. Though vertebrates as adults are rather di- 

 verse in structure and habits, they are all built according 

 to the same fundamental plan and pass through similar 

 stages in development. There is, therefore, no doubt that 

 vertebrates are genetically related ana, Decause their evolu- 



