282 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



in their distribution. Accessory gonads may be observed in various stages of forma- 

 tion, and they certainly appear, in this case, to have an ectodermal origin, being, so 

 far as one can see, primarily in connection with the epidermis. 



Bateson also thought it possible that the gonads were ectodermal in origin, 

 while Spengel considered it probable that the germ-cells arise in the first instance 

 in the mesenchyme. Morgan' states that " the gonad is formed from the mesoderm." 

 It seems not impossible that the gonads of the primary series may have a different 

 origin from the accessory gonads when the latter can be shown to be distinct neofor- 

 mations, as in the present species. 



The last gill-slits are quite dorsal in position (PI. XXXII. Fig. 56) and are followed 

 by a tract comparable to the postbranchial canal of species of Ptychodera but not 

 so well-defined. 



The dorsal submedian dermal tract below which the accessory gonads occur is 

 characterised both in the posterior branchial and in the genital regions by the attenua- 

 tion of the subjacent muscular layers. The circular muscles are not interrupted but 

 pass continuously across the tract ; the longitudinal muscular bundles are distinctly 

 interrupted, but at the same time straggling fibres are present in greater or less 

 numbers in the tract intervening between the dorsal and the ventro-lateral longitudinal 

 musculature ; so that the accessory genital ducts in this species may be said to 

 perforate the longitudinal musculature. 



Genito-hepatic Transition. ■ 



This crucial region is characterised in the present species by the occurrence of 

 intestinal canals and pores (Darmpforten of Spengel). Although there are nine 

 pores on each side they occupy a very short tract of the body because they tend 

 to overlap (PI. XXXII. Fig. 57). 



They lie in the submedian tract, i.e. in the line of the gill-pores, although 

 separated from the latter by the whole length of the genital region ; their superficial 

 resemblance to the last two or three pairs of gill-clefts is very striking. 



Each canal consists of an ectodermal involution which meets and fuses with an 

 outgrowth of the wall of the gut. The lumen of the canal appears more virtual 

 than actual (like that of the genital ducts). 



The first pore lies close against the submedian ridge and the succeeding pores 

 occur more and more lateral to this point until a maximum lateral deviation is 

 attained ; then the remaining pores successively approach once more the submedian 

 ridge until the last pore is in the same line with the first. The line of pores thus 

 describes an arc which is not in any way due to muscular contraction but is a 

 genuine anatomical feature. The pore-tract occurs at the posterior end of the genital 

 region immediately preceding the hepatic region. It lies a few millimetres in front of 

 the point denoted by an asterisk in PI. XXVII. Fig. 9, in fact it lies almost in the 

 middle of the sharp curvature which intervenes between the mid-hepatic region (denoted 

 by the asterisk) and the posterior dermal pits shown in the figure. The pore-tract 



1 T. H. Morgan, " The development of Balanoglossus,'' Joitrn. Morph. ix. 1894. See p. 60. 



