150 ESSENTIALS OF ZOOLOGY 



respiratory purposes, oxygen being absorbed and carbon dioxide be- 

 ing discharged from the blood here. There is no differentiation of a 

 distinct pharynx. 



The nervous system is composed of a dorsal cord which is tubular 

 in the region of the collar and extends the length of the trunk, a 

 more or less concentrated center of nerve cells in the collar, and a 

 ventral cord running longitudinally on the floor of the trunk. The 

 ventral cord certainly is not a chordate characteristic, but the domi- 

 nance and hollow structure of the anterior portion of the dorsal 

 one represent features which are homologous to the central nervous 

 system of higher chordates. 



These animals are dioecious, with gonads in the form of a genital 

 ridge extending lengthwise along each side of the anterior portion 

 of the trunk. The mature germ cells escape through the body wall, 

 are fertilized in the water, hatch out and become tornaria larvae, 

 which are globular in shape and form a pattern of ciliated bands 

 over the body. In this respect and in habit of life these larvae re- 

 semble the larvae of the echinoderms. On this basis a theoretical 

 relationship has been proposed. Until relatively recent times this 

 larva was mistaken for a form of adult nonchordate animal and 

 went under the genus name of Tornaria. 



Balanoglossus and other representatives of its subphylum, though 

 lacking in complete conformity to chordate characteristics, are classi- 

 fied here because of the diverticulum supposedly representing a imdi- 

 mentary notochord, the gill clefts in the alimentary canal, and the 

 dominance and grooved structure of the dorsal nerve cord. 



Subphylum Urochorda, Molg^la 



Subphylum Urochorda includes a number of common represen- 

 tative marine forms, such as Salpa, Cynthia, Ciona, Clavelina, As- 

 cidia, and Molgula. The latter genus represented by M. manhattensis 

 will be given particular consideration here. This animal is commonly 

 known as sea lemon, sea peach, or sea squirt. The body of the adult 

 is saclike and averages about one inch in diameter. In this condition 

 it would be an outcast among chordates because as an adult it has 

 no notochord, and no dorsally located, tubular nerve cord. How- 

 ever, it does present pharyngeal gill slits. M 



It is saved to the chordates by the presence of all three of the 

 characteristic features in the larval stage. The larva is free-swim- 

 ming and shaped like a tadpole, while the adult is globular and sessile 



