238 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



continued as a low ridge immediately outside of and below the liver-saccules, through 

 about four-fifths of the hepatic region (PI. XXVIII. Fig. 1). This mode of termination 

 of the genital pleurae is, in the main, characteristic of the subgenus Chlamydothorax. 

 Inseparably associated with the genital pleurae are the lateral septa which carry blood- 

 vessels to the gonads. So far as Spengel's account of them in Pt. erythraea goes they 

 have the same proximal origin and distal insertion in Pt. flava (PI. XXVIII. Figs. 6 

 and 7). But Spengel does not state the important fact that, whereas in most cases 

 the lateral septa are limited in their anterior extension, in Chlamydothorax (judging by 

 Pt. flava) they are coextensive, in front as well as behind, with the genital pleurae. 



This fact explains the meaning of the lateral septa as vascular folds of the base- 

 ment-membrane accompanying the pleural expansions or outgrowths of the body. 



In the branchial region of Pt. tiara the lateral septa arise from the basement- 

 membrane of the epidermis on the medial side of and dorsal to the angle formed 

 by the junction of the genital pleurae with the body-wall. Peripherally they are in- 

 serted again into the basement-membrane of the epidermis at the free edges of the 

 genital pleurae. Behind the pharynx their line of origin is transferred from the basal 

 membrane of the epidermis to the basement-membrane of the wall of the gut. In 

 the mid-hepatic region their line of origin occurs alternately in the side of the hepatic 

 diverticula, and in the basement-membrane of the epidermis in the intersaccular 

 intervals (PI. XXIX. Fig. 14). 



In a form like Pt. minuta with reduced genital pleurae the causal relations of 

 the lateral septa and genital pleurae are obscured, and the portion of the coeloni 

 enclosed by the septa appears as a pouch on each side ending coecally in front and 

 was so interpreted by Spengel. Pt. flam shows conclusively that the lateral septa belong 

 to the pleural system, and no pouching whatever (apart from the pleural outgrowths) of 

 the body-cavity has taken place. This should be particularly noted, as this is a tangible 

 example demonstrating how that Pt. minuta is a misleading form to take as a basis 

 for the interpretation of the Enteropneustic organisation (cf. Spengel, Man. p. 60). 



Branchiogexttal Transition. 



The region of the body which lies between the branchial and the hepatic regions 

 has been called the genital region by Spengel ; but as the gonads generally extend 

 for a greater or less distance into the branchial region, he also applies the term 

 branchiogenital to the two regions taken in combination. The intimate relation 

 between branchial and genital regions, which is expressed in this word, is probably of 

 great significance, and will be dealt with in the portion of this memoir devoted to the 

 morphology of the Enteropneusta. 



There is no true genital region in Pt. flava in the sense in which it occurs in 

 other forms, since the gonads are emancipated from the main body of the animal, being 

 confined to the genital pleurae. It is only therefore in comparison with other forms 

 that the short stretch of body which is intercalated between the posterior end of 

 the pharynx and the anterior end of the hepatic region, can be spoken of as the 

 genital region (cf. PI. XXVIII. Fig. 2). 



