386 HEMICHORDA [CH. 



generally calls up the idea of a vertebrate. Nevertheless the 

 number of species is not much more than half that of the Mollusca 

 and is not a tenth that of the described species of Arthropoda. 



Sub-phylum I. HEMICHORDA. 



The most primitive members of the phylum are certain worm- 

 like forms and certain sessile animals, in which it has taken special 

 research to discover traces of Vertebrate structure. The sessile 

 forms obtain their food like the Polyzoa by means of currents 

 produced by cilia clothing arborescent outgrowths of the body. 

 The worm-like forms live in mud, passing it through their intestines 

 and extracting nutriment from the organic matter it contains : 

 thus they feed and move forwards by the same process. Both 

 groups are marine. These animals are termed Hemichorda 

 (Gr. fri, half) on account of the short rudimentary notochord 

 which they possess. Sometimes they are called Enteropueusta 

 (Gr. Tn/ev/x-a, breath) because, like all Vertebrata, they use the 

 anterior portion of the gut for breathing. There are two orders : 

 the Balanoglbssida, which are worm-like and burrow in mud, and 

 the Cephalodiscida, which are sessile and live in clear water and 

 construct tubes for themselves in which they live, and superficially 

 resemble Polyzoa. The Balanoglossida are divided into several 

 genera, Dolichoglosstis, Chlamydothorax, Glossobalanus and others; 

 these genera have resulted from the splitting up of the old genus 

 Balanoglossus, which is sometimes still used as a semi-popular 

 designation for any member of the order. "We shall describe a 

 Balanoglossid as a type of the Hemichorda. 



The body of the animal is divided into three portions : (1) a 

 conical anterior part in front of the mouth, termed the proboscis; 

 (2) a swollen cylindrical portion immediately behind the mouth, 

 termed the collar; and (3) a long trunk, at the end of which is the 

 anus (Fig. 182). 



The proboscis contains one, and the collar and trunk each a 

 pair of special sections of the coelom or body cavity. The coelomic 

 sacs of the proboscis and collar communicate with the exterior by 

 ciliated tubes, termed the proboscis and collar pores respec- 

 tively (Figs. 183 and 184). The cilia lining these tubes produce 

 currents setting inwards: thus the collar and proboscis are kept 

 swollen up and tense with water and form efficient burrowing 

 instruments. If one of the Balanoglossida be removed from the 



