266 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



The fact that the first pair of genital pores occurs between gill-pores V. and 

 VI. again illustrates what I shall speak of later as the recession of the gonads 

 from the anterior end of the trunk. 



Ova. 



I was fortunate in obtaining a quite mature female, whose gonads contained vast 

 numbers of ripe ova. The eggs are enveloped in a stout vitelline membrane which 

 closely hugs the unfertilised ovum. The ovum is round and small, and its contents 

 are finely granular. 



The germinal vesicle contains a large, usually marginal nucleolus, which has great 

 attraction for eosin, and contains, one or many, fatty inclusions in its centre. In 

 the unstained condition these refringent inclusions in the nucleolus are exceedingly 

 prominent. 



The diameter of these eggs is "09 mm., and this is a measurement of great im- 

 portance because it is sufficient to inform us, I think with certainty, that the species 

 (like all Ptychoderidae) develops indirectly through a Tornaria-stage. 



The mode of oviposition of the Enteropneusta does not seem to be perfectly 

 understood [cf. Spengel, Mon., p. 658], and observations which I have made on Pt. 

 ruficollis are therefore of interest. Although the gonads are connected to the skin 

 by so-called ducts, Bateson thought that the eggs were discharged by rupture of the 

 body-wall as they are in many Annelids 1 . As shown in Plate XXXII. Fig. 69, at the 

 time when the ova are ready to be discharged the genital duct, in this species, becomes 

 properly hollowed out, and gapes widely enough for the passage of the ova without any 

 squeezing. 



Genito-hepatic transition. 



Sometimes the genital ridges can be traced for some distance into the hepatic 

 region ; sometimes they stop short some distance in front of the hepatic region. The 

 noteworthy point about the transition internally, is the occurrence of a longitudinal 

 ciliated tract on the left side about midway between the dorso-lateral margin of the 

 body and the ventral nerve-cord. This ciliated band is partly overhung by a fold of 

 intestinal epithelium, and appears to be comparable to the ciliated apparatus in the 

 gut of other Ptychoderidae. 



I was not able to ascertain how far this band extended into the abdominal region. 

 The post-genital portion of the body of this species is almost impossible to preserve 

 intact as the body-wall is very thin and brittle. Only the caudal region always pre- 

 serves its integrity. 



The wall of the gut in the hepatic region is thrown into numerous sacculations 

 throughout its entire circumference. These do not normally produce lobes of the body- 

 wall which would be visible externally. 



1 Possibly the discharge of the large eggs in the Balanoglossidae may be accompanied by rupture. 



