THE ANIMAL KINGDOM 1 7 



the pre-oral lobe, and upon the relations of the blood vessels and nerves. 

 Segmental excretory organs are, however, absent. 



There are possibly 50 species. 



BALANOGLOSSUS, the best-known genus, may be taken as representa- 

 tive. The body of balanoglossus is worm-like, and is divided into five 

 regions, proboscis, collar, gill region, "liver" region, and intestinal region. 

 The proboscis is a hollow muscular organ with an opening, a pore, on the 

 left dorsal side of the neck. The mouth lies on the ventral side between 

 the proboscis and the collar. The collar, like the proboscis, contains a 

 division of the coelom, which opens to the exterior by a pair of pores 

 near the mid-dorsal line. Like the proboscis, the collar also is muscular, 

 and used by the organism as a means of burrowing in the sand where it 

 lives. 



The pharynx is divided into a dorsal portion that contains the numer- 

 ous gill-apertures and a ventral portion which functions as the digestive 



BALANOGLOSSUS. t^tOUTH GILLS 

 Fk;. II. — lialanoiilossiis, a ty7-)ical genus of the suVj-jjhylum Hemichorda. (Redrawn 



after Bate.son.) 



passage of the pharynx. Posterior to the pharynx, the body contains a 

 series of gonadic sacs, each of which has a pore-like opening to the exterior. 

 The sexes are separate. 



In the so-called liver region, the intestine shows a series of paired 

 diverticula, each of which produces a corresponding bulging of the rela- 

 tively thin body- wall. These diverticula are glandular and supposed 

 to have a digestive function, hence their name. Behind the liver-region, 

 the intestine passes without convolution directly to the posteriorly 

 situated anus. The circulatory system resembles that of annelids, but is 

 supplemented by a lacunar system of lymph spaces. ''Fig. 532) 



The nervous system consists of dorsal and ventral nerve strands 

 containing occasional giant nerve cells. There are no special sense organs. 

 The so-called notochord is a diverticulum of the intestine which extends 

 with a narrow lumen into the proboscis from a point just behind the 

 mouth. 



The larva of Balanoglossus, known as Tornaria, shows rather striking 

 resemblances to the larva of echinoderms. As in echinoderms, the 

 blastopore becomes the anus. The sub-phylum, therefore, is included 

 in the group of Deuterostomia. 



Cephalodiscus and Rhabdopleura are genera which show resemblances 

 to Balanoglossus but which have a U-shaped alimentary canal. Rhabdo- 

 pleura is without gill apertures. (Fig. 536) 



