286 CLASSIFICATION OF ANIMALS 



geal chamber a short oesophagus leads to a stomach into which 

 the ducts of a digestive gland open. There is then a short 

 coiled intestine which opens into the atrium. There is a heart 

 but the vascular system is not well developed. The nervous 

 system consists of a single ganglion lying between the mouth 

 and atriopore. The sense organs are not well developed. All 

 Tunicata are hermaphroditic. The larva develops into a 

 "tadpole" which shows marked vertebrate affinities. Extend- 

 ing through the tail and for some distance into the body of the 

 "tadpole" is a rod of large cells which forms a supporting axis. 

 This is the notochord. On its dorsal side is a long tubular 

 nervous system which ends in front in a vesicle containing an 

 eye and a statocyst. At the anterior end of the body there are 

 three glandular papillae by which the tadpole finally attaches 

 itself. A metamorphosis then takes place in which the chorda 

 entirely disappears and the nervous system is reduced to the 

 single ganglion. The entire tail is resorbed and by a twisting 

 of the body in the further process of development the mouth 

 comes to lie opposite the point of attachment. 



622. Reproduction also takes place asexually and in many 

 forms colonies are formed by budding. The colonies may be 

 free swimming. 



623. Class III. Thaliacea. The Thaliacea are free swim- 

 ming, transparent, colonial forms with an alternation of gen- 

 erations. Solitary individuals give rise to a colony of sexual 

 individuals by budding and the colonial individuals produce 

 eggs which develop into the sexless solitary form. 



624. PHYLUM XI. Acrania. This phylum has so much in 

 common with the vertebrates that the absence of a skull was 

 considered of sufficient note to be indicated in the name of 

 the phylum. The body is elongated, flattened laterally and 

 pointed at both ends. There is a persistent notochord, a long 

 series of gill slits and a dorsal tubular, nervous system. The 

 body is segmented but there are no paired appendages. 



