4i6 



SUB- PHYLUM HEMICHORDA. 



There are about 30 species in 9 genera, e.g. Balanoglossus^ 

 Ptychodera, Schizocardium^ and Glandiceps. They are very 

 widely, though locally, distributed, and most occur in the 

 littoral area. 



a 



DESCRIPTION OF BALANOGLOSSUS. 



Form and habitat. The species which form this genus 



are worm - like marine 

 animals, found burrow- 

 ing in sand and mud in 

 almost all seas. They 

 vary in length from 

 about i in. to over 6 

 in., and are brightly 

 coloured and have a 

 peculiar odour, like that 

 of iodoform. The sexes 

 are distinct, and are 

 marked externally by 

 slight differences in 

 colour. The body con- 

 sists of a prominent 

 pre - oral region or 

 " proboscis," a firm 

 " collar " behind the 

 mouth ; behind this, a 



region with gill-slits ; and finally, a long, soft, slightly coiled 

 portion. 



Skin and muscles. --The epidermis is ciliated, and exudes 

 abundant mucus from unicellular glands. With the addition 

 of grains of sand, the mucus sometimes forms a tube round 

 the body. Some species are phosphorescent. The muscular 

 system is best developed about the proboscis and collar, 

 which are used in leisurely locomotion through the soft 

 sand. There are external circular and internal radial and 

 longitudinal muscles. The fibres are unstriped. There is 

 great regenerative capacity. 



Nervous system. The dorsal nerve - cord is most 

 developed in the collar, but is continued along the whole 

 length. It arises as a longitudinal groove of ectoderm and 



FIG. 205. Male of Balanoglossus (Do- 

 lichoglossus] kowalevskii. 'After Bate- 

 son. 



Note anterior proboscis. JMo., Mouth; op., 

 slight operculum behind the collar ; then the 

 region with gill-slits ; fs., testes ; a., anus. 



