228 exteropneusta from the south pacific, 



Colour, Measurements, and External Form. 



The colour is a nearly uniform dull yellow, sometimes with a tendency to a brownish 

 tinge. By attentive examination in the fresh condition, it is often possible to dis- 

 tinguish the males from the females by the presence of true brown pigment in the 

 integument covering the testes on the inner surface of the genital pleurae. The yellow 

 monotone is relieved in the hepatic region. The anterior hepatic coeca have a dark, 

 greenish-brown colour ; the mid-hepatic coeca are light brown, passing posteriorly into 

 the yellow ground-colour. 



The average length of the entire animal, when extended, may be taken at about 

 5 inches, the larger specimens attaining a length of 7 — 8 inches. It is a delicate, 

 fragile species. 



The most salient character of its external appearance is due to the presence of 

 the enormous pleural lappets, the genital pleurae. 



As is usual with Ptychoderidae, the proboscis is normally somewhat shorter than 

 the collar; the latter measures on the average 6 — 7 mm. in length. 



Pt. flava exhibits remarkable variation in respect of the length of the branchial 

 region. As a rule in the living animal the length of the perforated pharynx is about 

 equal to that of the proboscis and collar taken together, but it may be both shorter 

 and longer. Two extreme forms are met w r ith, namely, brachybranchiate and macro- 

 branchiate forms. The specimens which I obtained from the islet of Amedee (Isle 

 du Phare), some ten miles out from Noumea, inside the barrier reef, include a striking 

 series of brachybranchiate forms. Those from the Isle of Pines are on the whole inter- 

 mediate between the two extremes, while the macrobranchiate variety was found on 

 the weather side of the island of Lifu 1 . 



According to my measurements of numerous preserved specimens, those from the Isle 

 of Pines range from about 6 mm. to about 15 mm. in length of branchial region. Specimens 

 from Lifu range up to as much as 29 mm. in length of branchial region, the combined 

 length of proboscis and collar in this case measuring about 12 mm. When the genital 

 pleurae of such an individual are spread out nearly flat, so as to completely expose 

 the long, perforated pharynx, it forms a most striking object (see PL XXVI. Fig. 2). 



The length of the branchial region is of systematic importance in the case of 

 other species, and the variation of Pt. flava in this respect is therefore of particular 

 interest, since, according to the opinion which I expressed in my previous account of 

 this species, it appears, on the whole, to represent the most primitive Enteropneustic 

 type at present known. 



The postbranchial portion of the trunk intervening between the branchial and 

 hepatic regions is, as a rule, somewhat shorter than the branchial region, very 

 much shorter in the macrobranchiate forms. In most Enteropneusta this intermediate 

 tract constitutes the genital region proper, but cannot be so designated in this and 

 in allied species, because the gonads are entirely restricted to the genital pleurae. 



1 Individuals which have regenerated the anterior portion of the body resemble the brachybranchiate 



variety. 



