CHAPTER XX 



( Top to bottom ) salp, lancelet and sea squirt 



The Invertebrate | 

 Chordates 



{Fhylu/N C/tordaia) 



I 



F it were not for the existence of sea squirts, salps, 

 and lancelets, the phylum Chordata would consist 

 only of vertebrate animals — those with a vertebral 

 skeleton or backbone. But sea squirts, salps, and 

 lancelets do exist. This fact necessitates a broader 

 view of the qualifications for membership in the 

 phylum Chordata. 



This subdivision of the animal kingdom takes its 

 name from the notochord, a stiflfening rod of charac- 

 teristic construction serving as the first, inner, skeletal 

 support of the body. A notochord consists of a fibrous 

 sheath around a multitude of translucent cells whose 

 turgid condition provides firmness with flexibility. 



Possession of a notochord prevents a chordate's 

 body from telescoping as an earthworm does when its 

 longitudinal muscles contract. Instead, a notochord- 

 bearing creature bends from side to side, undulating 

 as a fish does. Lancelets retain the notochord through- 

 out life, whereas sea squirts, salps, and vertebrates 

 possess one only during larval or embryonic stages of 

 development. No member of any other phylum has a 

 notochord. 



Above its notochord, a chordate has a tubular 

 dorsal nerve cord, which may be enlarged at the an- 

 terior end into a true brain. This part of the animal 

 arises in a uniform manner in all chordates from sea 

 squirt to man, a procedure quite unlike any of the 

 ways in which a nervous system develops in mem- 

 bers of any other phylum. 



Chordates also show a third feature, found else- 

 where only among acorn worms (hemichordates) : a 

 series of openings between the pharynx region of the 

 digestive tract and the outside of the animal. Gill slits 

 of this kind are used throughout life by acorn worms, 

 sea squirts and salps, lancelets, and such vertebrates 

 as lampreys and fish. The tadpole stages of amphib- 

 ians use gill slits. Reptiles and warm-blooded verte- 

 brates possess pharyngeal clefts only during embry- 

 onic development. 



The invertebrate chordates include a few hundred 

 members of the subphylum Urochordata (sea squirts, 

 pyrosomes, and salps) and about thirty difi'erent kinds 

 of lancelets in subphylum Cephalochordata. These 

 names refer to the fact that in urochordates the 

 notochord is restricted to the tail region, whereas 

 in cephalochordates it extends to the anterior end of 

 the body. 



Invertebrate chordates resemble one another in 

 being exclusively marine and in possession of a pe- 

 culiar pocket, the atrium. Water from the pharynx 

 passes through the pharyngeal slits into the atrium, 

 and then to the outside world through a permanent 

 opening, the atrial pore. This current of water is 

 maintained by cilia on most inner surfaces of the 

 pharynx. It brings particles of food which become 

 entrapped in a film of mucus secreted over the same 

 pharyngeal surfaces. Cilia also move the loaded mu- 

 cus into a groove along the length of the pharynx 



286] 



