444 



GEPHYREA 



CHAP. 



Th. mellita was so named by Conn because it is found 

 sheltering in the Echinid Mellita. " It enters the shell at 

 the oral opening while yet very small, but once within its house 

 it grows to its adult size, and is obliged therefore to remain 

 during the rest of its life a prisoner." Each shell thus inhabited 

 acquires a reddish brown horse-shoe-shaped marking, which 

 affords a conspicuous signal that the shell contains a Thalassema. 



Thalassema is seldom found living in sand, and Bonellia never, 

 but Echiurus is almost always found in U-shaped tubes or 

 passages in the sand, which it digs out for itself by the rapid 

 contractions of its body-wall aided by its bristles. It, like the 

 other two genera named above, does not long remain in the same 

 hole, but frequently changes its home. As a rule the Echiurus 

 sits near the mouth of its tube, which is often a foot or even 

 two in depth, and sends out its proboscis in every direction ; at 

 the least sign of disturbance it withdraws into the deeper recesses. 

 The walls of the tube are kept from falling in by a layer of 

 mucus, which makes a smooth lining to the passage. The peri- 

 anal bristles, which can be withdrawn or protruded at will, 

 enable the animal to fix itself at any level in the tube. 



The Echiuroidea are sometimes used by fishermen as bait. In 

 Echiurus pallasii Greef found three parasites, all of them new 

 species. One, a Gregarine, he named Conorhynchus gibbosus ; the 

 others were Platyhelminthes, and were named by him Distomum 

 echiuri and Nemertoscolex parasiticus respectively. 



IV. Order Epithetosomatoidea. 



This Order includes the single Family Epithetosomatidae, which 

 was established by Koren and Danielssen to contain the re- 

 markable Gephyrean they described in 1881 under the name 

 Epithetosoma norvegicum (Fig. 225). 



Unfortunately only two specimens were at their disposition, 

 and these were badly preserved, so that many details of their 

 structure could not be made out. The animals are of an olive- 

 green colour, and consist of a trunk about 12 mm. long, and 

 of a proboscis 30 mm. in length ; the latter differs essentially 

 from the proboscis of the Echiuroidea inasmuch as it is hollow, 

 and seems to be a whip -like tubular extension of the skin. 

 Its lumen opens into the body -cavity. Ventral to the base 



