WITH NOTES ON THE WEST INDIAN SPECIES. 



267 



Abdominal-caudal transition. 



The ventral nerve-cord in the abdominal region lies at the base of a groove which 

 comes to an abrupt termination at the junction of the abdominal and caudal regions 

 (PI. XXVII. Fig. 7). 



The surface of the body in the caudal region is smoother than in the abdominal 

 region, and the dermal annulations more regular. 



The body-wall in the abdominal region is flaccid and highly collapsible, while the 

 caudal region is always well distended. The rigidity of the caudal region is no doubt 

 partly due to the circular musculature, and partly to the presence of a stout pygo- 

 chord (PI. XXX. Fig. 35). The ventral dilated edge of the pygochord consists of a 

 cord of large cells, each with a central nucleus from whose neighbourhood radiating 

 strands of protoplasm pass to the periphery of the cell, strongly reminding the observer 

 of the axial cells of the tentacle of a Campanularian hydroid. Similar cells with 

 stellate contents occur in the superjacent constrictions of the pygochord. 



Musculature of Body-wall. 



Pt. ruficollis agrees with Pt. hedleyi Hill in the absence of circular muscles from 

 the body-wall of the trunk except in the caudal region. At the anal extremity they 

 combine with the muscularis of the gut -wall to form a sphincter ani. 



OECOLOGY. 



Pt. ruficollis lives commensally with Pt. carnosa in so far that it inhabits the 

 burrows of the latter. At the volcanic island of Matupi in Blanche Bay (New Britain) 

 when one investigates the Enteropneustic castings with the aid of a spade it is 

 almost an even chance whether one will unearth the one species or the other, 

 although Pt. carnosa is the predominant form. At the coral islet known to the local 

 whites as Pigeon Island and to the native blacks as Palakuvur Pt. ruficollis is the 

 predominant form or at least it is the one which is easier to procure. 



Both these species are victims of autotomy ; but whereas Pt. carnosa is thick and 

 fleshy, Pt. ruficollis is thin and brittle. 



