440 SUB-PHYLUM HEMICHORDA. 
Affinities with Vertebrates (especially emphasised by Bateson). 
(1) ‘* Motochord.”—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, Vcz.). 
It may be compared with the notochord of Vertebrates, 
which also arises dorsally from the gut. But it lies delow 
the main dorsal blood vessel, is of very limited extent, and 
may be merely an analogue of the notochord. 
(2)  Gell-séits.’—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 
vesemble 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.7.). 
(4) ‘* Zhe celom.”—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 cesophageal pockets opening to 
the exterior in four Annelid types—Polygordius, Saccoctrrus, 
Spio fuliginosus, 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. (1) Cephalodiscus 
Cephalodiscus dodecaiopaus 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. 
