20 COMPARATIVE ANATOMY 



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 

 hypophysis of vertebrates. The unpaired eye and static organ contained 

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



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 Hne of vertebrate ancestry seems 

 not unlikely. 



Sub-Phylum Cephalochorda (Acrania) 



The c'ephalochordates 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 

 are limited to the gill region. As in urochordates, the gills open into a 

 peribranchial cavity. Development involves metamorphosis. (Fig. 539) 



There are possibly 25 species. 



AMPHIOXUS. The lancelet, amphioxus, the characteristic genus of 

 the group and the so-called connecting hnk between vertebrates and 

 invertebrates, interests morphologists because of its resemblance to the 

 hypothetical ancestor of vertebrates. If the amphioxus had not been 

 discovered, it must have been invented for theoretical reasons. Its 

 resemblance to the larva of cyclostomes is impressive. 



The Amphioxus is a lance-shaped animal, not more than two inches 

 long, with a laterally compressed body and a median caudal fin. Its 

 external orifices are an anterior ventrally placed mouth, an anus to the 

 left of the caudal fin, and an atriopore somewhat behind the middle of the 

 body. The atrial chamber which surrounds the elongated pharynx is 

 formed by the union of paired lateral folds which meet in the mid-ventral 

 Une of the body. Such a structure seems to be an adaptation to the 

 sand-burrowing habit of the adult animal. The atrial cavity ends bUndly 

 in front, and opens externally by the atriopore just behind the pharynx. 

 In the region of the pharynx, a pair of ventro-lateral metapleural folds 

 extend as far back as the atriopore. 



