>v 



180 ARTHUR WILLEY. 



elements which compose the branchial skeleton, namely, pri- 

 mary or septal rods, secondary or tongue-rods, and synapticula 

 or cross-rods, were developed at a time and in a type in which 

 their presence was absolutely necessary to prevent the collapse 

 of the branchial sac. It is not so easy to see that their 

 presence is directly necessary to those forms in which the 

 septa between adjacent gill-slits are fused with the thick 

 parenchymatous tissue of the body-wall, and the slits only" 

 open to the exterior by minute pores. But they are there not- 

 withstanding, namely, because they are derived from forms in 

 which the presence of skeletal supports for the much-perforated 

 pharyngeal wall was a sine qua non. Such a form is P. 

 flava, with its free and otherwise unsupported pharynx. 



If so much is admitted, then the presence of the genital 

 pleura, covering over the unprotected pharynx, needs no 

 special comment. 



Thirdly, the fact that in Schizocardium and in Glandiceps 

 Hack si the anterior region of the alimentary canal is not 

 subdivided into branchial and oesophageal portions militates 

 strongly against these genera being regarded as more primitive 

 than Ptychodera. 



Finally, the habitat of Ptychodera in the littoral zone, 

 often between the tide-marks, is another positive indication of 

 the primitive character of the genus. The greatest depth 

 recorded by Spengel for a species of Ptychodera is 20 feet 

 for P. minuta in the Bay of Rio de Janeiro. Schizocardium 

 (S. brasiliense, Spengel) descends to 18 to 20 fathoms; 

 Glandiceps (G. talaboti, Marion) descends to 450 metres, 

 while another species (G. abyssicola, Spengel) was obtained 

 by the " Challenger '' from 2500 fathoms; Balanoglossus 

 Kupfferi, von Willemoes-Suhm, was obtained from 12 to 16 

 fathoms. 



It is very possible that the forms which have migrated into 

 deeper water may have retained some primitive features which 

 are lost to the littoral or tidal forms, just as many of the 

 Elasipoda among the Holothuroidea have retained the 

 primitive connection of the stone-canal with the exterior, 



