CHAPTER XVII 
PHYLUM CHORDATA 
SUB-PHYLUM HEMICH ORDA 
UNnbeER the title Hemichorda are included a number of 
interesting types which seem to have affinities with Verte- 
brates. These affinities are clearest in certain worm- 
like animals with distinct gill-clefts, eg. Balanoglossus 
and tychodera, which form the class Enteropneusta. 
Perhaps allied to these are two peculiar types,—Rhabdo- 
pleura and Cephalodiscus, which may be united in the class 
Pterobranchia. Very doubtfully in this alliance is Phoronis. 
GENERAL CHARACTERS OF ENTEROPNEUSTA 
The worm-like body has three regions—a pre-oral “ pro- 
boscis,” a “collar” around and behind the mouth, and a 
trunk, the anterior part of which bears gill-slits. A dorsal 
and in part tubular nerve-cord arises from the ectoderm along 
the middle line, and ts connected, by a ring round the pharynx, 
with a ventral cord. In the skin, which is covered with 
ciliated ectoderm, there 1s also a nerve plexus. From the 
anterior region of the gut a diverticulum grows forward for 
@ short distance, becomes a firm support for the proboscis, and: 
7s often called the “notochord.” The gill-slits open dorsally, 
are very numerous, and increase in number during life. The 
mesoblast is formed by the outgrowth of five caelom pouches 
Jrom the -archenteron. An unpaired anterior pouch forms 
the pre-oral or proboscis cavity of the adult; there are two 
collar cavities and two trunk cavities. 
There are about 30 species in 9 genera, ¢.g. Balanoglossus, 
