CHAPTER XVII. 



PHYLUM CHORDATA. 

 SUB-PHYLUM HEMICHORDA. 



UNDER the title Hemichorda are included a number of 

 interesting types which seem to have some affinities with 

 Vertebrates. 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, Rhabdopkura and 

 Cephalodiscus, which may be united in the class Ptero- 

 branchia. Still more doubtfully in this alliance is Phoronis, 

 which almost requires a class for itself (Phoronidea). 



GENERAL CHARACTERS OF ENTEROPNEUSTA. 



The body is divisible into three regions a pre-oral 

 "proboscis" a "collar" around and behind the mouth, and 

 a trunk, the anterior part of which bears gill-slits. A dorsal 

 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 solid support for the proboscis, and is often called the 

 " notochord." The gill-slits open dorsally, are very numerous, 

 and increase in number during life ; in some details of 

 development they recall those of Amphioxus. The mesoblast 

 is formed by the outgrowth of pouches from the archenteron ; 

 i.e. the body cavity is enteroccelic. An unpaired anterior 

 pouch forms the pre-oral or proboscis cavity of the adult, 

 and is compared to the anterior unpaired body cavity of 

 Amphioxus. 



