XIII 



PHYLUM CHORD AT A, 



diate allies ; the other, the Plero- 

 branchia, including Cephalodiscus 

 and Rhabdopleura. 



External Characters and 

 Coelome of Enteropneusta. 

 Balanoglossus (Fig. 705) is a soft- 

 bodied, cylindrical, worm-like animal, 

 the surface of which is uniformly 

 ciliated. It is divisible into three 

 regions ; in front there is a large 

 club-shaped hollow organ the pro- 

 boscis (pr.) ; immediately behind the 

 proboscis and encircling its base is a 

 prominent fold the collar (co.) ; the 

 third region or trunk is long and 

 nearly cylindrical, but somewhat 

 depressed. 



Balanoglossus lives in the sea, bur- 

 rowing in sand or mud by means of 

 its proboscis : one species has been 

 found swarming on the surface of the 

 sea. Numerous glands in the in- 

 tegument secrete a viscid matter to 

 which grains of sand adhere in such 

 a way as to form a fragile temporary 

 tube. The proboscis (Fig. 706, prob.) 

 has muscular walls ; its cavity (pro- 

 boscis-coelome) opens on the exterior 

 usually by a single minute aperture 

 the proboscis-pore (prb.po.) rarely 

 by two. In some species the pro- 

 boscis-pore does not communicate 

 with the proboscis coelome, but ter- 

 minates blindly, and may send off a 

 narrow tubular diverticulum which 

 opens into the neuroccele. The nar- 

 row posterior part or " neck " of the 

 proboscis is strengthened by a layer 

 of cartilage-like or chondroid tissue, 

 which supports the blood-vessels. The 

 collar is also muscular, and contains 

 one cavity, or two (right and left) 

 separated from one another by dorsal 

 and ventral mesenteries, and com- 

 pletely cut off from the proboscis- 

 cavity. The collar-cavity and also 

 that of the proboscis are crossed by 

 numerous strands of connective-tissue 



gen. 



FIG. 705. Balanoglossus. En- 

 tire animal, br. branchial region ; 

 co. collar ; gen. genital ridges ; hep. 

 prominences formed by hepatic cseca; 

 pr. proboscis. (After Spengel.) 



B 2 



