BALANOGLOSSIDA 653 



History.— T\\\^ group was first formed by Gegenbaur in 1870, who 

 placed it among the worms. When, however, the fundamental researches 

 of Kowalevsky became known and their significance appreciated the 

 relationship of the group to the Tunicata and Amphioxus was universally 

 recognized, and in 1884 it was brought under the Chordata by Bateson, 

 who at the same time proposed the name Hemichorda for it. 



Order BALANOGLOSSIDA. 



Elongated, Avorm-like Enter opneusta, which are often common on sand 

 fiats. The proboscis is a cylindrical or ovoid structure joined with the collar 

 by a narrow neck. It contains a portion of the coelom which opens to the 

 outside by a dorsal pore (5), or in some cases by two; the collar also 

 contains two similar cavities, each with its pore. The trunk has an 

 extensive coelom in the form of 

 a pair of elongated cavities be- 

 tween which lies the digestive 

 tract (9), supported by a dor- 

 sal and a ventral longitudinal 



mesentery. This tract is straight Fig. 99G — ^Diagram of a longitudinal sec- 



tion of Balanoglossus (Shipley and Mac- 



and extends from the mouth at Bride), l, proboscis; 2, collar; 3, trunk; 



4, heart ; 5. proboscis pore ; 6, nervous 



tl:e anterior end of the collar system ; 7, dorsal blood vessel ; 8, bran- 



chial region of intestinal tract ; 9, pharyn- 



to the anus at the hinder end geal region; lO, gill slits; ll. ventral 



blood vessel ; 12, mouth ; 13, notochord. 



of the body: from its anterior 



end a dorsal finger-shaped diverticulum (13), which represents the 

 notochord, extends forwards into the proboscis. The entire anterior 

 portion of the intestinal tract is placed in direct communication with 

 the exterior by a series of paired gill slits (10), back of which in certain 

 genera is a series of paired liver sacs which show conspicuously on the 

 outer surface of the body. A dorsal (7) and a ventral (11) blood ves- 

 sel accompanies the intestine, the former of which passes above the 

 notochord and ends in a sinus (4) situated in the collar and proboscis. 

 The nervous system is mostly sub-epithelial and diffuse. Mid-doi-sally in 

 the collar and mid-ventrally in the trunk, however, are definite aggrega- 

 tions of nerve cells and fibers which are joined by a ring nerve just 

 behind the collar. The portion of the dorsal strand (6) in the collar is 

 tubular in shape and distinct from the outer epithelium and arises as a 

 neural canal, like the central nervous system of vertebrates: in certain 

 genera it is still joined with the outer epithelium by a series of median 

 nerves called nerve roots. 



The sexes are separate, the gonads appearing as paired pouches along 

 the branchial region and back of it, each pouch opening to the outside 



