490 HEMICHORDA 



the middle line, and is connected, by a ring round the pharynx^ 

 with a ventral cord. In the skin, which is covered with 

 ciliated ectoderm, there is also a nerve plexus. From the 

 anterior region of the gut a diverticulum grows forward for 

 a short distance, becomes a firm support for the proboscis, and 

 is often called the " notochord." The gill-slits open dorsally, 

 are very numerous, and increase in number during life. The 

 mesoderm arises by the outgrowth of five coelom pouches 

 from the archenteron. An unpaired anterior pouch forms 

 the pre-oral or proboscis cavity of the adult ; there are two 

 collar cavities and two trunk cavities. 



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

 Dolichoglossus , Ptychodera, Schizocardium, and Glandiceps . 

 They are very widely, though locally, distributed, and most 

 occur in the littoral area. 



Description of Balanoglossus 



Form and habitat. — The species which form this genus 

 are worm-like marine animals, burrowing 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 consists of a prominent turgid and muscular " pro- 

 boscis," a firm " collar," 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 de- 

 veloped in the collar, but is continued along the whole 

 length. It arises as a longitudinal groove of ectoderm and 

 often remains tubular, like a typical spinal cord. The 

 dorsal nerve-cord is connected by a band round the collar 



