390 CEPHALOCHORDA [CH. 



The Cephalodiscida exhibit the same three regions of the body 

 as the Balanoglossida, but these are very differently developed. 

 The proboscis becomes a flat glandular shield bent down over the 

 mouth. It secretes the material out of which the house or tube 

 is constructed. The collar region is prolonged into hollow arms 

 which are beset by rows of ciliated tentacles which waft microscopic 

 swimming organisms into the mouth. The central nervous system 

 remains as a plate of exposed ectoderm. The trunk is short and 

 rounded and the anus is shifted on to the dorsal surface. There 

 is never more than a single pair of gill-pores, and in minute forms 

 these may be absent. From the ventral surface springs a stalk 

 from which buds are produced which grow up into new persons. 



Prof. Gilchrist has shown that the individuals of a Cephalodiscid 

 colony can emerge from their tubes and creep out. They fix them- 

 selves to the substratum by the ventral surface of their praeoral lobes 

 or proboscides exactly as the larva of an Asteroid does. (See p. 366.) 



Cephalodiscus with twelve arms and one pair of gill-pores is fairly 

 abundant in the Antarctic and Southern Atlantic Oceans. 



Rhabdopleura, with two arms and no gill-pores, which is found 

 fairly abundantly in moderately deep water in the North Sea, was 

 formerly described as a Polyzoon. The absence of gill-pores in 

 Rhabdopleura is no doubt a secondary modification due to the very 

 small size of the individuals, for we find that as the size of animals 

 of the same build is diminished, respiratory organs tend to disappear. 



One point of interest attaching to the Hemichorda is that 

 they may commence life as free-swimming larvae, resembling the 

 larvae of the Echinodermata, and suggesting the thought that 

 perhaps two such different groups as the Vertebrata and Echi- 

 nodermata may have descended by different paths from the same 

 simple free-swimming ancestors. 



In dealing with the larvae of Echinodermata it was pointed out 

 that the middle section of the coelom called the hydrocoele, which 

 develops into the water-vascular system of the adult, has been 

 compared to the collar coelom of Hemichorda. This comparison 

 is supported by the condition of the collar region in Cephalodiscida, 

 where, as we have seen, it is produced into outgrowths very like the 

 radial canals of Echinodermata and beset with tentacles which may 

 be compared to tube feet. 



Sub-phylum II. CEPHALOCHORDA. 



Leaving the Hemichorda we next come to some small fish-like 

 animals, the Cephalochorda, which were formerly all included 

 under the name Amphioxus, and indeed there is no very strong 

 reason for breaking up this old genus. The name Amphioxus (a//,</>i 



