168 ARTHUR WILLEY. 



the more opaque regions of the proboscis and collar. Some- 

 times the body has a more brownish tinge. The anterior liver 

 sacs, however, offer a relief to the general yellow ground 

 colour, in that they are of a dark greenish-brown colour, while 

 the sacs about the middle of the hepatic region are of a light 

 brown, passing posteriorly into the usual yellow colour. The 

 liver sacculations pass quite gradually behind into the ordinary 

 annulations of the body-wall, and it is not always easy to say 

 which is the last hepatic diverticulum. 



In cases where the body has evidently been broken in two 

 in the hepatic region, and the anterior portion of the body, 

 including the whole of the branchial region has been lost at 

 no very distant period, a new collar and proboscis have been 

 added by regeneration immediately in front of the liver-sacs, 

 while the branchial region would no doubt be regenerated later. 

 In such regenerated individuals the collar and proboscis are 

 white and unpigmented. 



The proboscis in the normal condition is distinctly grooved in 

 the dorsal middle line, and in this respect P. flava may be 

 compared with Balanoglossus sulcatus, Spengel (cf. fig. 1). 

 The liver-sacs are not always simple smooth outgrowths, but 

 the larger ones are distinctly lobed, and sometimes present a 

 digitate appearance. An intensification of this lobed structure 

 would probably lead to such a diffuse arrangement of the 

 liver-sacs as is met with in P. erythroea, Spengel. 



In only two individuals out of the many that have passed 

 under my observation have I observed them to be infested with a 

 curious parasite (? Ive balanoglossi, Paul Mayer), originally 

 remarked by Spengel in P. minuta, and more recently by 

 Hill in P. australiensis. As described by Hill in the latter 

 species, the parasite occurs in one of the genital pleurae (in the 

 example here figured on the right side), where it forms a very 

 prominent tubular enlargement " (cf. fig. 2) . Hill states that 

 in P. australiensis " a large proportion of the individuals" 

 are infested with the parasite. 



