496 



HEMICHORDA 



it becomes smaller and more opaque, the ciliated bands become less 

 marked, it sinks to the bottom and metamorphosis sets in. It acquires 

 a proboscis ; the collar becomes marked off, and on its dorsal side a 

 groove closes to form the beginning of the nerve cord. Tongue bars are 

 formed, and the gill pouches open by degrees to the exterior, making 

 gill-slits. The " notochord " grows out from in front. There is an 

 elongation of the post -oral region, and the body begins to lengthen 

 to the worm-like adult form. The Tornaria must be regarded as the 



more primitive larval state ; the 

 temporary absence of mouth and 

 anus in the direct development is 

 probably an adaptation acquired 

 after the pelagic habit was lost. 



Johannes iVIuller ranked the 

 Tornaria larva, whose adult form 

 was not then known, beside the 

 larvae of Echinoderms. The ciliated 

 bands of the Tornaria resemble 

 those of Echinoderm larvse, but 

 this is only a superficial character- 

 istic. The anterior pouch, which 

 forms the cavity of the proboscis 

 and communicates with the ex- 

 terior, has also been compared with 

 the beginning of the water-vascular 

 system in Echinoderms, and it is 

 true that in both several independ- 

 ent coelom pouches grow out from 

 Fig. 282.-Tornaria larva, from ^^^^ Primitive gut The anterior 

 the side.-After Spengel. ^^^^ ^f ^^^ ^^ Balano gloss us com- 



municates with the exterior by a 

 m., Mouth ; g. gut ; a., anus; h., heart ; poj-e, which becomes the proboscis- 

 p., pore entering proboscis cavity; ^ , ,, , ,, , ^, . , 



c.r., anal ringof cUia ; s.c.r., secondary Pore of the adult, and this has been 

 anail ring. The dark wavy line in- compared with the water-pore, or 

 dicates the margm of the lobes of the outlet of the water-vascular system 



larval body wath their bands of cilia. ^r x?^\.- i ™ i,- -u • ■ 



Note also "the apical spot with cilia ^f Echinoderms. which opens simi- 

 and sense organ (fl.s.). larly from an anterior enteroccel to 



the exterior. 



Affinities with Vertebrates (especially emphasised by Bateson). 



(i) ''Notochord." — A dorsal outgrowth from the anterior region 

 of the gut grows forward for a short distance into the pro- 

 boscis, and becomes a solid supporting rod (Fig. 281, Nch.). 

 It may be compared with the notochord of V^ertebrates, 

 which also arises dorsally from the gut. But it lies below 

 the main dorsal blood vessel, is of very limited extent, and 

 may be merely an analogue of the notochord. 



(2) " Gill-slits." — Numerous gill-slits (Fig. 278) open from the 

 anterior region of the gut to the exterior, and are separated 

 from one another by skeletal bars, which in some ways 

 resemble the framework of the respiratory jiharynx in 



s.cv- 



