302 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



which accompany the genital pleurae and carry the genital blood-vessels. They mark out 

 the path of emancipation of the gonads from the gill-slits 1 . 



In other existing forms, excepting Bal. canadensis' 2 , the genital pleurae have under- 

 gone progressive reduction (from before backwards) with the result that the gonads 

 have become to a greater or less extent secondarily restored to the main body of the 

 animal and the gill-slits have been closed in laterally, their external openings being 

 reduced to minute pores placed dorsally on each side of the dorsal nerve-cord. 



The gonaducal (submedian, Spengel) line is that in which the genital pores are 

 placed and it coincides with the series of gill-pores except in those Ptychoderidae 

 which possess genital pleurae. The gonaducal line of the Ptychoderidae is further 

 denoted by the peripheral insertion of the lateral septum into the basement-membrane 

 of the epidermis. This line in Chlamydothorax occurs at or near the dorsal free edge 

 of the genital pleurae. In Tauroglossus, where the gonads lie partly in the pleural 

 folds and partly in the main body-cavity, the gonaducal line has shifted from the summit 

 nearly to the base of the genital pleurae 3 . Finally in such a species as Pt. ruficollis, 

 where there are no genital pleurae in the branchial region, the gonaducal line and 

 branchial groove coincide. 



Having been secondarily restored to the main body-cavity the gonads have appa- 

 rently been exposed to the influence of another set of changes tending to their further 

 limitation to a definite genital region. 



I refer to the recession of the gonads from the branchial region. In com- 

 paratively few forms do the gonads extend to the extreme anterior end of the 

 branchiogenital region. They do so in Pt. minuta and Pt. Jlava and a few others ; 

 but as a rule they commence at various distances from the posterior rim of the 

 collar. This is best shown in tabular form. 



Table showing Recession of the Gonads. 



SPECIES GONADS 



Pt. flava Coextensive with genital pleurae. 



Pt. ruficollis First genital duct between 5th and 6th gill-pores. 



Pt. sarniensis First genital duct beside the 20th gill-pore, about 4 mm. behind collar. [Spengel.] 



Pt. aurantiaca Gonads commence some millimetres behind collar; in branchial region they are 



arranged in a single series and in the genital region in multiple series. 



[Spengel.] 



Sch. brasiliense Gonads commence about level of 15th gill-pore. [Spengel.] 



Sch. peruviamim Gonads not present in fragment in which TO pairs of gill-clefts occurred. [Spengel.] 



Gl. hacksi Gonads extend throughout entire branchial region and genital region into hepatic 



region. In young specimen they commenced at the level of 13th — 14th 



gill-pores. Thus the most anterior gonads of adult are added secondarily. 



[Spengel.] 

 Gl. abyssicola Gonads commence behind the branchial region. In this case alone is the 



recession complete. [Spengel.] 



1 Just as recurrent nerves show divergence from primitive topographical relations. 



2 Bal. canadensis, as described by Spengel, is a most interesting and peculiar species. It possesses two 

 sets of pleural folds which contain the gonads, dorsal and ventral. The genital blood-vessels of this form 

 constitute a system sui generis, and there are no lateral septa. 



s This shifting of the gonaducal line (cf. PI. XXVHI. Fig. 6 and PI. XXX. Fig. 23) may also be re- 

 garded as evidence of the possibility of change of function of the genital pleurae (cf. p. 317). 



