Classification of Chordates: Protochordata 101 



of vertebrates. The reproductive organs lie in the loop of the intestine, 

 posterior to the stomach. Their ducts extend forward and open into 

 the atrial cavity near the anus. The gonads are hermaphroditic. There 

 are no excretory tubules. 



The nervous system consists of a ganglion or "brain," derived from 

 the anterior end of the larval nerve-cord. It lies in the body-wall be- 

 tween the two apertures of the body. Ventral to the brain is a "neural 

 gland" which has been compared to the ueural part of the pituitary 

 gland of vertebrates. The single eye and the static organ of the larva 

 degenerate during metamorphosis. 



Some tunicates, although anatomically similar to the sessile "sea- 

 squirts," do not become attached but are free-swimming in the adult 

 stage, propelling themselves by forcible ejection of water from the 

 atrial aperture. Such a form is Doliolum, having a barrel-shaped body 

 with oral and atrial openings at opposite ends. Salpa resembles Doli- 

 olum (Fig. 310). 



Of the three or four orders of Urochorda, the Larvacea are of 

 special interest because they undergo no metamorphosis. The adult 

 retains the larval tail with its notochord and spinal cord and is actively 

 free-swimming. Appendicularia, one of the Larvacea, resembles a 

 very minute tadpole (Fig. 311). 



That Appendicularia is a persistent, primitive, free-swimming tuni- 

 cate — i.e., one in whose ancestry metamorphosis has never occurred — 

 is an interesting possibility. But it is also possible that it is what might 

 be described as a "persistent larva" — i.e., a descendant of metamor- 

 phosing ancestors, but with loss of the metamorphosis and accmisition 

 of capacity for reproducing at a structural stage corresponding to that 

 of the larva of its metamorphosing ancestors. 



SUBPHYLUM CEPHALOCHORDA 



This subphylum includes the fishlike little "lancelet," Amphioxus, 

 first named Branchiosloma (Fig. 312), and a few closely similar animals 

 — perhaps 20 or more species in all. Amphioxus may attain a length of 

 40 to 50 mm. They are all marine, living in the shallow coastal waters 

 of both Eastern and Western Hemispheres. Amph ioxus burrows in clean 

 sand and commonly rests in the sand with merely the head exposed. 

 It swims very actively and. curiously, with the body in a vertical 

 position. 



The cephalochordates resemble vertebrates much more closely than 

 do the other protochordates. The adult has a well-developed and func- 

 tional notochord extending from tip of head to tip of tail. Dorsal to 

 it is a tubular nerve-cord, slightly enlarged at its anterior end to form 

 a so-called "brain." The respiratory arrangement strongly resembles 



