CHAPTER XVII. 



CLASS HEMICHORDA OR ENTEROPNEUSTA. 

 Type. BALANOGLOSSUS. 



A SPECIES of Balanoglossus was described by Delle Chiaje 

 at the end of the eighteenth century, but it is only within 

 the last few years that the researches of Spengel, Bateson, 

 and others have led to an appreciation of the importance of 

 this type, and to a recognition of its peculiar features. 



The class (Enteropneusta) which was erected for the re- 

 ception of Balanoglossus is now known to include a few 

 other forms, whose more or less distinct affinities with Ver- 

 tebrates are suggested by the alternative title Hemichorda. 

 Taken along with Tunicates and Amphioxus, they afford 

 interesting illustrations of the gradualness with which Ver- 

 tebrate characters have appeared in history. 



GENERAL CHARACTERS. The body is divisible into three 

 regions a pre-oral "proboscis" a "collar" around and be- 

 hind the mouth, and a trunk, the anterior part of which bears 

 gill slits. A dorsal nerve cord arises from the epiblast along 

 the middle line, and is connected, by a ring round the pharynx, 

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

 ated 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 un- 

 paired anterior pouch forms the pre-oral or proboscis cavity 



