THE ANIMAL KINGDOM 9 



canal consists of a short esophagus, a spherical stomach, and an intestine 

 which leads to an anus situated well forward in the atrial chamber. 



The heart lies ventral to the esophagus in the pericardial chamber. 

 There are no closed blood-vessels, but the blood is pumped from the heart 

 forward to the pharynx in lacunar spaces the relations of which resemble 

 those of the afferent branchial vessels of vertebrates. The reproductive 

 organs He 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. 



The nervous system consists of a ganglion or brain, which lies in the 

 body-wall between the two apertures of the body. Ventral to the brain 

 is a neural gland which has been compared with the neural part of the 

 pituitary gland of vertebrates. The unpaired eye and static organ con- 

 tained in the brain vesicle of the larva degenerate in the metamorphosis. 



STATIC ORGANx 



ENOOOERM STRAND 



PHARYNX'' 

 CILL SLITS 



OCELLUSf l^f^r 



Fig. 10.— Diagram of a larval urochordate. The similarity of the larval uro- 

 chordate to the embryo of a cephalochordate (Amphioxus) suggests that a form like 

 this lies near the main line of vertebrate ancestry. (Redrawn after von Beneden and 

 Julin modified.) 



Ciona during its ontogenesis undergoes a striking metamorphosis, 

 which indicates that the animal is a degenerate descendant of a primitive 

 branch of the chordate tree. 



Of the four orders of urochordates the Larvacea are of special interest 

 since they develop without metamorphosis, and hence show no sign of 

 degeneration. Their caudal appendage contains a notochord and spinal 

 cord. That they lie close to the main line of vertebrate ancestry seems 

 not unlikely. 



Sub-Phylum Cephalochorda (Acrania) 



The cephalochordates are those chordates in which the notochord 

 occurs not only in the head, as in Hemichorda, or in the tail, as in the 

 Urochorda, but throughout the entire length of the body. The group is 

 sometimes called the Acrania because, as the name suggests, a brain case 

 is lacking. Metamerism is strikingly manifested in the muscles and 

 nerves, which form an unbroken series from the tip of the snout to the tip 

 of the tail. Segmental protonephridia are metamerically arranged, but 



