CHAPTER XVII 



PHYLUM CHORDATA 

 SUB-PHYLUM HEMICHORDA 



UNDER 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, e.g. Balanoglossus 

 and Ptychodera, 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 is connected, by a ring round the pharynx, 

 with a ventral cord. In the skin, which is covered with 

 ciliated ectoderm, there is also a nerve plexus. From the 

 anterior region of the gut a diverticulum grows forward for 

 a short distance, becomes a firm support for the proboscis, and 

 is 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 cozlom pouches 

 from 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, e.g. Balanoglossus, 



