Classification of Chordates: Protochordata 407 



possibility of propulsion by forcible ejection of water from the atrial 

 siphon —the skyrocket principle or "jet propulsion." Doliolum passes 

 through i< transitory larval stage in which I here is a characteristic tail 

 and notochord. Salpa omits I his stage. Development in both involves a 

 highly complex alternation of sexual and asexual generations. The 

 vertebrates afford numerous instances of this reversion to an ancient 

 mode of life, but without reversion to the ancient anatomy. Externally , 

 the flipper of a seal looks like a fin, but its internal structure is that of a 

 land leg. 



SUBPHYLUM YERTEBRATA 



In the now living Vertebrata, the notochord, present in all em- 

 bryos, persists in the adult in a functional condition (in association 

 with some rudimentary vertebral structures) only in the round- 

 mouthed eels (Cyclostomata) and in certain fishes (Holocephali, stur- 

 geons, and lungfishes or Dipnoi). In all other vertebrates it is function- 

 ally replaced by a vertebral column composed either of cartilage or of 

 bone. Pharyngeal clefts, present in all embryos, persist in the adult 

 stage of such vertebrates as are gill-breathing throughout life (fishes 

 and some amphibians). In all other vertebrates the clefts are restricted 

 to the embryonic period, except in so far as those of the most anterior 

 pair may persist in relation to the adult ear. The central nervous 

 organ is dorsal and permanently hollow and expands at its anterior 

 end into a more or less massive and complex brain. A protective skele- 

 tal brain-case, the cranium, composed either of cartilage or of bone, is 

 developed around the brain and in intimate relation to the three im- 

 portant sense-organs, the olfactory organ, eye, and ear. In distinction 

 from the Acrania (Cephalochorda), the name Craniata is often used 

 as the equivalent of Vertebrata. 



The several other characteristic features of vertebrates have been 

 set forth in Part I. 



A mere glance at the great and highly diversified group of verte- 

 brates reveals the fact that their most conspicuous anatomic differ- 

 ences are those which relate to the external medium with which the 

 animal is directly in contact — water, land, air. The animal parts which 

 are most immediately put to the necessity of being adapted to the 

 nature of the medium are the locomotor organs, the organs of breath- 

 ing, and the skin. In such a typical aquatic animal as a fish, the paired 

 appendages are fins, breathing is by gills, and the skin is coated by a 

 thin layer of mucus ("slime"). In the typical land vertebrate, the 

 paired appendages, adapted for walking and running on a hard sub- 

 stratum, are jointed legs having a normal maximum of five digits 

 (fingers or toes) on each — therefore pentadactyl appendages. Breath- 



