Classification of Chordates: Protochordata 



395 



coelom/ ^--/. 



OF PROBOSCIS - »/ i-/ M ouTH 



LONG MUSCLEsY I'NOTOCHORD ""("""tOLLAR 



OF PROBOSCIS UoBOSdS SKELETON 



Fig. 304. Anterior half of Balanoglossus, seen, in left lateral aspect, as a semi- 

 transparent object. (After Stempell. Modified from Neal and Rand: "Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



hollow and muscular. These muscular structures are used for burrow- 

 ing into the sand or mud in which the animal lives — usually between 

 low and high tide levels. 



In the anterior part of the trunk, the digestive tube is differenti- 

 ated into a respiratory region or pharynx which is divided into a dorsal 

 portion containing the many paired gill-apertures (40 to 50 or more 

 pairs in old animals) and a ventral alimentary passage. In the more 

 posterior region of the trunk, the intestine has a series of paired 

 pouches, each of which causes a corresponding bulge of the relatively 

 thin body-wall. These pouches are glandular and presumably have ;i 

 digestive function; hence they are called "liver" diverticula. Behind 

 this "liver" region, the intestine passes directly to the posterior anus. 



The central nervous system consists of a dorsal nerve-cord to- 

 gether with a ventral nerve-cord similar to that of an invertebrate. 

 They are connected with one another in the region of the collar. The 

 dorsal cord in this region is hollow to an extent varying in different 

 species. There are no locally specialized sense-organs. 



The so-called notochord is a short, tubular diverticulum of the ali- 

 mentary tube extending into the proboscis from a point just behind the 

 mouth (Fig. 304). It apparently serves to strengthen the base of the 

 proboscis and is therefore presumably skeletal in function. 



The circulatory system resembles that of annelid worms. 



The reproductive organs consist of a series of paired gonadic sacs 

 situated some in front of and some behind the junction of the pharynx 

 and the "liver" region of the intestine. Each sac has a pore opening to 

 the exterior. The sexes are separate. 



Balanoglossus and some related genera constitute a group called 

 Enteropneusta. Other hemichordates are Cephalodiscus and Rhab- 

 dopleura, which show resemblances to Balanoglossus, but are short- 

 bodied and have a U-shaped alimentary tube. Rhabdopleura lacks gill- 

 slits. Both are sessile, living either at shallow or deep sea-levels. Rhab- 

 dopleura forms colonies. 



