440 SUB- PHYLUM HEMICHORDA. 



Affinities with Vertebrates (especially emphasised by Bateson). 



(1) " 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. 236, Nch.}. 

 It may be compared with the notochord of Vertebrates, 

 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. 234) 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 pharynx in 

 Amphioxus. There are, however, many differences in 

 detail, thus the slits open dorsally, not laterally ; the 

 skeletal bars are differently disposed ; the blood supply is 

 different. 



(3) " Dorsal nerve-cord" A dorsal median insinking (Fig. 235, 



d.n.) of ectoderm, especially strong in the region of the 

 Collar, may be compared with the medullary canal of Verte- 

 brates. But it must be noticed that there is also a ventral 

 nerve-cord (Fig. 235, v.n.). 



(4) " 77ie ca'/ot/i." The development of five enteroccelic pouches 



is very suggestive of affinities with Amphioxus. 



Affinities with Annelids (after Spengel). 



The larva (Tornaria) (Fig. 237) may be regarded as a modified 

 Trochosphere, but this points at most to a far-off common 

 stock. Moreover, the nephridia, usually present in the 

 Trochosphere, are unrepresented in the Tornaria. 



The heart lies, as in some Annelids, dorsal to the gut, not 

 ventral as in Vertebrates ; the dorsal vessel carries blood 

 forwards, the ventral backwards, as is usual in Annelids. 

 But the double nervous system is essentially different from 

 that of Annelids ; and the gill -slits are unrepresented there, 

 though Salensky has described oesophageal pockets opening to 

 the exterior in four Annelid types Polygordius^ Saccocirrus, 

 SpiofuliginosuS) and Polydora cornuta. In the last there are 

 five pairs in the larva, and two persist. If there be a 

 relationship between Enteropneusta and Annelids, it must 

 be a very distant one, perhaps restricted to origin from some 

 common stock. 



Class PTEROBRANCHIA. (i) Cephalodiscus 



Cephalodiscus dodeca.ophu* was dredged by the Challenger in the 

 Magellan Straits. Others are known from Japan, the Malay Archi- 

 pelago, South Africa, and the Antarctic. It was at first described by 

 M'Intosh as a divergent Polyzoon, but the researches of Harmer point 

 to relationship with Balanoglossus. 



