20 THE SEAS 



tubes of lime, sand or parchment-like material which they 

 make for themselves, enlarging them as they grow and being 

 in many cases able to make new ones if they are destroyed. 

 Closely related are the leeches of which there ara marine 

 species which suck the blood of fish, and rather less so the 

 Sipunculids which have a protrusible proboscis and a tough 

 leathery body without any sign of segmentation. They 

 are all marine. One of the commonest constituents of 

 the drifting life of the sea is the little arrow worm, Sagitta, 

 which belongs to a small group called the Ch^tognatha, 

 which has no connection with the other worms. 



Quite closely related to the bristle-worms are the Polyzoa 

 or sea mats, minute creatures which always live in colonies, 

 some of which are small and branching like the hydroids, 

 and others large and with strong limy skeletons which give 

 them the appearance of delicate corals. In spite of their 

 minute size the individual animals are very complex. 



The Echinodermata are a diverse group, exclusively 

 marine, which include the starfish and sea urchin. Like 

 the Ccelenterates, they are built on a radial plan though, 

 owing to their mode of life, some have altered this and become 

 bilaterally symmetrical. They all have limy skeletons, 

 some consisting of continuous plates forming a compact 

 shell as in the sea urchins, and others of isolated spicules 

 or tubercles embedded in a leathery skin, as in the sea- 

 cucumbers. They are in most instances either attached 

 or very slowly-moving animals, the mechanism of loco- 

 motion being usually provided by the peculiar " tube- 

 feet," tiny tubes which are worked by water power supplied 

 by a series of canals within the body. There are five 

 distinct groups, the starfish (Asteroidea) with a flat central 

 disc to which are attached a number of arms, usually five, 

 though it may be many more ; the brittlestars (Ophiur- 

 oidea) not unlike the starfish but with the disc more 



