i66 ZOOLOGY 



ventral vessel. Sipunculus and Phascolosoma, have remarkable 

 bodies known as " urns " floating in their coelomic fluid. They 

 are bell-shaped structures, with a ring of cilia round the mouth, 

 and a nucleus. These remarkable corpuscles are formed by the 

 division of certain large cells on the wall of the dorsal blood- 

 vessel, they were formerly thought to be parasitic Infusoria. 



The Achaeta have no special organs of locomotion, and 

 probably do not move about much. Sipunculus and Phas- 

 colosoma usually live half embedded in the sand, which they 

 swallow in large quantities. Phascolion lives in empty worms' 

 tubes or iu mollusc shells, and its body is often permanently 

 twisted, accommodatiag its shape to that of its home. Pliymo- 

 soma lives in holes or passages in coral rock, or in holes 

 between stones. As a rule the members of this subdivision 

 occur only in comparatively shallow water. 



The Gephyrea chaetifera are provided with a pro- 

 stomium, which may acquire enormous proportions. In Ponellia 

 it may, when fully extended, attain a length of 2 or 3 feet, 

 whilst the body is only 1^ to 2 inches long. In this genus it 

 is bifid at the end. In Echiurus, BoTielUa, and Thalassema 

 there are a pair of large chitinoid hooks placed anteriorly on 

 the ventral side of the body, and in some species of JEchiurus 

 there is one, sometimes two, posterior circlets of setae, each 

 seta originating from a single cell, like those of the Chaetopods. 



Bonellia viridis is coloured a bright green by a pigment 

 termed " bonellein/' which is not identical with chlorophyll. 

 The mouth in the Chaetifera lies at the base of the pro- 

 stomium, which is ciliated and grooved, and is doubtless used 

 to catch minute organisms for food; the iatestiae is looped 

 and the anus terminal. In Bonellia, Echiurus, and Thalassema 

 a " siphon " or collateral intestine, such as is found in the 

 Capitellidae and Echinids, is present. 



Branched organs open into the rectum in most of the 

 Chaetifera. At the end of each branch is a small funnel- 

 shaped ciliated opening leading into the coelom. The ceUs 

 lining the tubes of these branches have been seen crowded 

 with excretory granules, and they may possibly function as 

 nephridia as well as serve to regulate the amount of fluid in 

 the coelom. 



