170 ARTHUR WILLEY. 



genital region of Spengel) of lateral wing-like expansions, 

 which can be folded over so as to meet one another in the 

 dorsal middle line, and so to completely embrace the branchial 

 region and the most anterior portion of the hepatic region. 

 These are the genital wings (Genitalfliigel) of Spengel, so 

 called because they contain the gonads. We may conveniently 

 refer to thera as the genital pleurae. 



In P. flava the genital pleurae have a very low origin, 

 arising from the ventro-lateral margins of the body, and they 

 constitute remarkable structures. They are very mobile, and 

 in life can be spread out laterally nearly flat; while, as already 

 stated, they can meet over the pharynx in the mid-dorsal line, 

 thus producing a most effective peripharyngeal cavity or 

 atrium, opening to the exterior posteriorly in the neighbour- 

 hood of the anterior hepatic region. The genital pleurae of P. 

 flava attain their maximum development within the branchial 

 region, and maintain it for some distance into the post-bran- 

 chial region, behind which they gradually decrease in size, and 

 finally die out on the outer sides of the liver-sacs (fig. 1). In 

 P. australiensis, according to Hill, they reach their maxi- 

 mum size somewhat posterior to the gill region. 



The Pharynx (cf. Fig. 3). 



When, in the living animals kept under observation, the 

 genital pleurae are spread out laterally, a complete and 

 beautiful view of the entire pharynx is to be obtained. The 

 latter is then seen to stand up, erect and independent, in the 

 middle of the peripharyngeal area, and the branchial bars are 

 visible nearly if not quite throughout their whole length. 

 Dorsally, on either side of and adjacent to the dorsal nerve- 

 cord, two whitish pigmented bands extend throughout the 

 length of the pharynx. These are the bands which in most 

 Enteropneusta form the inner or median boundary of the lon- 

 gitudinal grooves into which the gill- pores open. 



In P. flava, however, the U-shaped gill-slits open freely to 

 the exterior throughout their whole extent, and their external 

 openings are therefore not reduced to minute circular or ellip- 



