Fig. 217.—Cloeosiphon aspergillum Quatr. × ½. a, Introvert covered with spines and partially extended, but not sufficiently to show the head; b, calcareous plates surrounding the point of origin of the introvert.

The genus Aspidosiphon includes nineteen species, which are, with few exceptions, exclusively confined to the Indian Ocean and neighbouring seas, including the Red Sea. The exceptions are A. armatus from the Norwegian coast, and A. mülleri from the Mediterranean and Adriatic. A. truncatus is also stated to occur at Panama, the Bahamas, and at Mauritius. The remaining species almost all occur in the Malay Archipelago and neighbouring islands, and as was the case with Phymosoma, this part of the world seems to be the headquarters of the genus. A. mülleri lives in the interstices of rocks and stones, and occasionally in disused Mollusc shells.

Two species of Aspidosiphon have been described by Bouvier[[484]] living in a state of commensalism with two species of Madreporarian corals, Stephanoceris rousseaui and Heteropsammia cochlea, which live on and surrounding the shells of certain Molluscs at Aden (Fig. 216). Apparently the Gephyrean takes up its abode within its house at a tender age, and according to Bouvier, it provides for its increasing bulk by secreting a coiled calcareous tube, the outer surface of which affords space for the growth of the coral.

The genus Cloeosiphon, the Echinosiphon of Sluiter, includes three species: C. aspergillum (Fig. 217), C. molle, and C. javanicum. The first named occurs at Mauritius, the Malay Archipelago, and neighbouring islands; the others are confined to the last-named area, which thus again forms the headquarters of a genus.

Golfingia, described by Lankester from a single specimen, was dredged in St. Andrews Bay, at the depth of 10 fathoms.

Petalostoma comprises but one species, P. minutum, which is found in the English Channel.

Onchnesoma comprises two species, O. steenstrupii and O. sarsii, both found off the coast of Norway at considerable depths between 200 and 300 fathoms.

Tylosoma comprises one species, T. lütkenii, also from the Norwegian coast. It is dredged from stony ground in 50 to 80 fathoms.

II. Order Priapuloidea.

Anatomy.—This Order consists of the two genera Priapulus and Halicryptus. Both are cylindrical animals with the mouth at one end and the anus at the other. The introvert is short, and is covered with rows of chitinous spines, which are continued to some extent over the body.