300 CHARACTERS OF VERTEBRATE ANIMALS 



The individuals of Fixed Ascidians are of comparatively 

 large size in the Simple forms (fig. 171), of which the example 

 taken is a typical one. The Colonial forms are made up of 

 smaller individuals, and the colonies produced may be of the 



most varying form and size, while the 

 degree of union between the members 

 of the colony is more or less complete. 

 In such a native genus, for instance, as 

 Clavellina, we find a creeping stalk-like 

 structure from which a number of indi- 

 viduals grow up, each of them being 

 Fig, ^^^.-Botryllus csscntially similar to Ascidia. A good 



To left, a small colony, showing groups . p . ^ . . • • r l 



of individuals. To right, two groups of example ol more mtimate union is tound 

 ''^lX'':^^':^-^^l^::iril in Botryllus (fig. 174)- which can often 



mouths at outer ends. In centre is an |-,g found at loW tide aS a blulsh Sfek- 



atriopore common to all of them. *~^ 



tinous crust upon stones and brown 

 sea- weed. Imbedded in this are star-shaped groups of small 

 individuals. 



Among Free-swimming Ascidians the only Simple forms are 

 Appendicularia (fig. 173) and its allies. The Colonial members 

 include the remarkable phosphorescent Pyrosoma, which is 

 shaped like a hollow cylinder closed at one end, and the Salps, 

 of which more will be said elsewhere. Both are common in the 

 Mediterranean. 



Sub-class III.— WORM-LIKE PROTOCHORDATES 



(Hemichorda) 



Here are massed together a small number of forms about 

 which there has been endless discussion, and of which the one 

 with most certain tenure of Chordate rank is a worm-like 

 creature which has no common name, but which may perhaps 

 be called the Acorn-headed Worm {Balanoglossus) (fig. 175). 

 It is found at low-tide mark in many parts of the world, living 

 in mud or sand which it glues together into a sort of temporary 

 tube by means of a slimy fluid poured out from the skin. One 

 species is found in the Channel Islands. The front of the body 

 is made up of a swollen proboscis, yellow or orange in colour, 

 and capable of altering its shape to a very great degree. It is 

 attached behind by a narrow stalk, and the general outline in 



