The position of the anus in the Sipunculoidea is one common to very many animals which live embedded in sand or in tubular holes; it is probably not primitive, as in the larva of Sipunculus it is near the posterior end, and becomes more dorsal as the larva elongates.
The anal vesicles of the Echiuroidea probably have no representative in the Sipunculoidea. In appearance and position they are very like the little tufts which are found on the rectum of Sipunculus, but since these open neither into the body-cavity nor into the alimentary canal, it is hardly fair to compare them.
The resemblances between the Orders seem to me, on the whole, to outweigh the differences. The general structure of the skin, the coiled alimentary canal, with its ciliated groove, supported by strands of muscles, the vascular system which gives off no capillaries, the structure of the brown tubes, the existence of chitinous hooks or bristles, the nervous system with its single unsegmented ventral cord, the formation of the generative organs, all point to a sufficiently close resemblance to justify us in classing the two Orders together. In addition to these there are considerable histological resemblances which cannot be discussed here, but which have a certain weight.
To sum up, it seems probable that the Echiuroidea are derived from the Chaetopoda, and that their nearest ally in this group is Sternaspis; and that the Sipunculoidea are allied to the Echiuroidea, but have further departed from the Annelid stock, and have lost even those traces of affinity with the parent group which have been preserved in the development of Echiurus and Thalassema.
So little is known of Epithetosoma that it is difficult to discuss its affinities. The presence of the hollow proboscis and the pores leading into the body-cavity undoubtedly justify its being placed in a separate Order, but beyond the presence of a terminal anus, in which it resembles the Echiuroidea, there is nothing in its structure which connects it more nearly with one than with the other of the three larger Orders of Gephyrea.
List of Gephyrea found in the British Area as defined by Canon Norman.
| Phascolosoma vulgare Blainv. | English Channel and North Sea. |
| Phasc"losoma elongatum Kef. | English Channel. |
| Phasc"losoma papillosum Thom. | English coast. |
| Phasc"losoma eremita Sars | North Sea. |
| Phasc"losoma procerum Moeb. | Bass Rock. |
| Phascolion strombi Mont. | English coast (Plymouth). |
| Sipunculus nudus L. | North Sea, English Channel (Paignton, Teignmouth). |
| Golfingia macintoshii Lank. | East coast of Scotland (St. Andrews Bay). |
| Petalostoma minutum Kef. | English Channel (Plymouth). |
| Priapulus caudatus Lam. | Scarborough, Outer Hebrides. |
| Echiurus pallasii Guérin | Coast of Scotland, English Channel. |
| Thalassema neptuni Gaertner | English Channel (Coast of Devonshire). |
CHAPTER XVI
PHORONIS
HISTORY—HABITS—STRUCTURE—REPRODUCTION—LARVA—METAMORPHOSIS—LIST OF SPECIES AND LOCALITIES—SYSTEMATIC POSITION.