3 88 



HEMICHORDA OR ENTEROPNEUSTA. 



is formed by the outgrowth of pouches from the archenteron ; 

 i.e. the body cavity is enteroccelic. An unpaired anterior 

 pouch forms the pre-oral or proboscis cavity of the adult, 

 and is compared to the anterior unpaired body cavity of 

 Amphioxus. 



Spengel, in his recent monograph, recognises nineteen 

 species and four genera — Balanoglossus, Ptychodera, Sckizo- 

 cardium, and Glandiceps. They are very widely, though 

 locally, distributed, but are perhaps absent on the Pacific 

 coasts of America. 



Description of Balanoglossus. 



Form and habitat. — The species which form this genus 



are worm-like marine 

 animals, found in sand 

 and mud in the English 

 Channel, the Mediter- 

 ranean, Chesapeake Bay, 

 etc. 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 exter- 

 nally by slight differ- 

 ences in colour. The 

 body consists of a pro- 

 minent 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. In B. robinii 

 the mucus sets firmly, and, with the addition of grains of 

 sand, 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 



Fig. 169. — Male of Balanoglossus kowa- 

 levskii. — After Bateson. 



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

 slight operculum behind the collar ; then the 

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



