174 CEYLON PEARL OYSTER REPORT. 



Systematic Position. 



Centrosiphon belongs to the same group of Sipunculoids as Aspidosiphon. and 

 Cloeosiphon. All three genera possess a posterior and an anterior shield, the latter 

 being placed but slightly in front of the anus and at the base of the introvert. 

 Centrosiphon differs from Aspidogaster in that its introvert emerges from the centre 

 of a round shield, quite symmetrically, and in a line corresponding with the long 

 axis of the body. In Aspidosiphon the anterior shield is shaped something like a 

 cockle-shell, and the introvert emerges at or near the narrow end and is at an anj-le 

 with the main axis of the body. Centrosiphon differs from Cloeosiphon in the absence of 

 the very characteristic calcareous ring round the base of the siphon, with its lozenge- 

 shaped areas, and in the absence of true hooks, though the latter are represented 

 by chitinized prominences. It resembles Cloeosiphon in its central proboscis and in 

 its continuous sheath of longitudinal muscles. In Aspidosiphon the sheath may or 

 may not be continuous. It differs from both genera in possessing four retractor 

 muscles, and in the fact that its anterior shield forms a flat platform and not a 

 radially or bilaterally symmetrical cone. 



Cloeosiphon aspergillum, Quatr. 



A single specimen, from Stn. XLIV., West of Pantura, 30 fathoms. This species 

 is common in the Indian Ocean and parts of the Pacific. The " Siboga " took this 

 form at thirteen stations, and in his account of the Gephyrea collected by the 

 Expedition on that vessel, Sluiter records his view that the variety javanicum 

 should be dropped. 



Physcosoma agassizii, Sel. and de Man. 



A single specimen was taken at Stn. LV., South-west ot Periya Paar, in the Gull 

 of Manaar ; depth, 11-14 fathoms. 



Physcosoma asser, Sel. and de Man. 



Three specimens ; one from the Pearl Banks off Aripu (Stn. LIV.), and another 

 from the Lagoon, Galle. Since my paper in the ' Fauna and Geography of the 

 Maldive and Laccadive Islands,' this form has been recorded by Sluiter from four oi 

 the "Siboga" stations. 



Physcosoma scolops, Sel. and de Man. 



Three specimens of this genus are described as coming "out of a scarlet-ball 

 sponge (Axinella tubulata), (Jheval Paar (Stn. IX.. 7 fathoms)," and also West of 

 Periya Paar, 12-14 fathoms, in the same sponge. The species is very common all 

 over the Indian Ocean from the Red Sea and the East Ooast of Africa to the East 

 Indies. 



