WITH NOTES ON THE WEST INDIAN SPECIES. 



265 



My deduction from the foregoing data is, that the postbranchial canal represents 

 what was formerly a greater posterior extension of the pharynx ; that it is, in fact, 

 the more or less metamorphosed relic of a portion of a primitively more extensive 

 perforated pharynx. In Pt. flava, as well as in Pt. carnosa, it is still the seat of 

 origin of new gill-clefts in the normal position. But in Pt. ruficollis, as well as in 

 Pt. hedleyi, it is emancipated from any connection with the gill-clefts. 



Genital Region. 



In Pt. ruficollis there is a true genital region in a sense in which it is not 

 present in Pt. flava. In the latter we saw that the gonads were emancipated from 

 the main body of the animal. In the present species the gonads are incorporated into 

 the body. 



The species is remarkable for the great length of the postbranchial genital region, 

 a peculiarity which it shares with Pt. sarniensis. There are no genital pleurae, but the 

 dorso-lateral margins of the body are squared off sharply behind the branchial region 

 and are continued backwards as longitudinal ridges between which the median dorsal 

 region is sometimes depressed. But often the body is subcylindrical in shape, the 

 prominence of the genital ridges no doubt depending upon the condition of the gonads 

 and also upon the state of muscular contraction of the body. 



The gonaducal lines (submedian lines of Spengel) in the genital region are seen 

 in the living animal to be continuous with the branchial grooves. 



The gonads encroach upon the branchial region, extending forwards (as shown in 

 one series) to the level of the first gill-pouch on one side, and to that of the 

 second gill-pouch on the other side. This difference of level of the anterior gonads 

 is merely due to differences in the amount of their lobation, since the gonads of 

 the first pair open approximately in the same plane between the fifth and sixth pairs 

 of gill-pores. 



The succession of the anterior seven genital ducts and their relation to the gill- 

 pores is shown in the following table ; a simple Roman numeral indicates that the 

 genital duct occurs beside the corresponding gill-pore ; two numerals connected by a 

 hyphen indicates that the genital duct occurs between two gill-pores. 



37—2 



