XVIII] 



METAMORPHOSIS 



407 



ganglion (6, Fig. 201). The neuropore 

 however persists and develops into a 

 mass of tubes underlying the ganglion, 

 which is called the sub-neural gland. 

 Its opening acquires a crescentic form 

 with thickened lips, and is called the 

 dorsal tubercle. Meanwhile the 

 chin grows enormously, so as to rotate 

 the mouth up and away from the sub- 

 stratum, and thus the long axis of the 

 pharynx becomes vertical instead of 

 horizontal. The skin of the region 

 where the anus becomes situated is 

 depressed so as to form a groove. 

 This becomes confluent with the outer 

 parts of the two gill-slits, so as to 

 form a single dorsal cavity termed the 

 atrial cavity, the opening of which 

 is not far from the mouth. It must be 

 noticed that this cavity does not 

 correspond to the similarly named 

 cavity in Amphioxus: in the case of 

 the last-named animal the atrial walls 

 originate from the dorsal edges of 

 the gill-slits and meet one another 

 beneath the animal; whereas in the 

 Urochorda they arise from the ven- 

 tral edges of the slits, and are united 

 with one another on the dorsal edge. 

 The gill-slits themselves become changed 

 by the growth of numerous partitions, 

 transverse to the axis of the pharynx, 

 into a series of narrow slits ; and then 

 by the formation of another series of 

 stronger bars parallel to the long axis, 

 into a veritable ciliated trellis-work. 

 All this trellis-work is sup- 

 ported by horny rods like 

 the gill-bars of Amphioxus. 

 The test thickens enor- 

 mously and becomes invad- 

 ed by a finger-like outgrowth 



10 



FIG. 202. Ciona intestinalisxl. The live 

 animal seen in its test ; some of the organs 

 can be seen, as the test is semi-transparent. 



1. Mouth. 2. Atrial orifice. 3. Anus. 

 4. Genital pore. 5. Muscles. 6. Stomach. 

 7. Intestine. 8. Beproductive organs. 

 9. Stalk attached to a rock. 10. Tentacular 

 ring. 11. Peripharyngeal ring. 12. Brain, 



