528 



HERMICHORDA OR ENTEROPNEUSTA. 



marks round our shores. They are known as Ascidians or sea-squirts, and 

 they may be either solitary or colonial. The solitary forms are sac-shaped, 

 leathery creatures fastened at the base, and possessing a mouth at the opposite 



Fig. 4. FYROSOMA. 



Fig. 5. COMMON SEA 



SQUIRT (Ascidlella, 



aspersa). 



Fig. 6. BOTRYLLUS. 



end. There is also another opening by which the water from the gill- 

 slits is passed out of the body. The colonial forms are smaller in size, and 

 are united together into irregular or star-shaped clusters usually brightly 

 coloured. 



CLASS VIII. HERMICHORDA or ENTEROPNEUSTA. 



THE commonest representative of this group is the remarkable Balanoglossus, 

 an animal which combines the creeping mud-burrowing habits of a worm with 

 some of *he organs especially characteristic of ver- 

 tebrata. The body is divided into three parts, a flexible 

 tongue-like proboscis in front of the mouth, a raised 

 collar behind it, and a long cylindrical hind-region, the 

 sides of which are perforated by numerous inconspicuous 

 gill -slits. In the back of the collar of 

 Balanoglossus a short tubular nerve-cord 

 is found like that of vertebrata. In its 

 young condition Balanoglossus is a small 

 transparent creature, about one-tenth of 

 an inch in length, remarkably similar to 

 the free-swimming larval form of star- 

 fishes and sea-cucumbers. 



Balanoglossus has only once been taken 

 on the coast of the British Isles, but is Fig. 8. TORNARIA. 

 not uncommon on the shores of the Channel Islands and 

 other parts of the world. Balanoglossus sarniensis, found in the Channel 

 Islands, varies from one to three feet in length, and is difficult to obtain in a 

 perfect condition, owing to its softness and fragility. The young form, 

 Tornaria, may be found in the autumn months free-swimming at the surface 

 of the sea off the southern shores of Great Britain. 



Fig. 7. BALANO- 

 GLOSSUS SARNIENSIS. 



