Fig. 224.—Thalassema neptuni Gaert. × 2. A, The animal lying on its ventral surface. B, Ventral view of the anterior end, showing the grooved proboscis ending behind in the mouth, and the ventral hooks.
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 The peri-anal bristles, which can be withdrawn or protruded at will, enable the animal to fix itself at any level in the tube.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 remarkable 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 of the proboscis is the mouth; the intestine is straight, and terminates in the anus, which is posterior. The nervous system lies between the circular and the longitudinal muscles of the body-wall, and contains a tube, the nature of which is obscure. No vascular system is known. The ovary is attached to a mesentery ventral to the anterior part of the alimentary canal, and there is a single nephridium. No anal vesicles exist.
The most remarkable feature of the genus is a series of pore-like openings, which are stated to lead from the outside into the body-cavity (Fig. 225, a). These are arranged four on each side, at the bottom of two slit-like depressions in the skin, which lie one on each side of the base of the proboscis, slightly dorsal to it.
These remarkable structures are without parallel amongst the Gephyrea, and, together with the peculiar character of the proboscis, justify the Norwegian naturalists in adding a new family to the group.
