420 . GEPHYREA chap. 



forerunners of the brown tubes, are found, and the chief muscle 

 tracts are already established. The nervous system is still in 

 close connexion with the skin, from the outer part of which it 

 is derived ; the cerebral thickening bears two eye-spots. 



The fluid of the body-cavity contains corpuscles, which are 

 kept in active circulation by the constant contractions of the 

 body-wall, and by numerous tufts of cilia which are borne on 

 the inner surface of the skin. The dorsal blood-vessel is one of 

 the latest organs to arise. 



The larva swims actively about for a month, during which 

 time it increases greatly in size ; it then undergoes a somewhat 

 sudden metamorphosis. The ciliated ring and the structures 

 related to the oesophagus begin to disappear, the distinction 

 between the head and the rest of the body is obliterated, and the 

 head becomes relatively small. The mouth changes its position, 

 and becomes terminal instead of being somewhat ventral, and 

 the tentacular membrane begins to appear. At the same time 

 the larva relinquishes its free-swimming life, and sinks to the 

 bottom ; it begins creeping amongst the sand by protruding and 

 retracting the anterior part of its body, and takes on all the 

 characters and habits of the adult. 



I. Order Sipunculoidea. 



Besides the genus Sipunculus, the Order Sipunculoidea includes 

 ten other genera. A key to these, taken for the most part from 

 Selenka's admirable monograph, is given on page 424. 



Phascolosoma contains, in comparison with Sipunculus, only 

 small species, and it is easily distinguished by the fact that the 

 longitudinal muscles are fused into a continuous sheath. As a 

 rule the skin is smooth. A few species bear hooks, which are 

 generally scattered irregularly and not arranged in transverse 

 rows, as in Phymosoma (Fig. 214) and most of the other genera. 



The fold which in S. nudus surrounds the mouth may be 

 in the same species bent in so as to take the form of a double 

 horse-shoe, the opening of which is always dorsal, just above the 

 brain ; in this case the mouth is crescentiform. In other genera 

 the fold is broken up into discrete tentacles, and these are vari- 

 ously arranged ; in Dendrostoma they are grouped together in 

 four or six bundles round the mouth, but the more usual arrange- 







