218 On the Genital Pouches in Thalasscnia neptuni. 



XXIX. — Noteona Variation in the Number of Genital Pouches 

 in Tliahissi'ina iioi)tuni, Gaertner. By F. H. SteWART, 

 I\1.A., Gatty Marine Laboratory, St. Andrews, 



AVniLE dissectino; a specimenof Thalissema neptuni^ Gaertner, 

 I obseivod tliat it varied from the normal in possessing five 

 genital pouches instead of four, one being placed in the 

 middle dorsal line, the remaining four lying, as usual, two on 

 each side. But on examination by means of sections the 

 pouch which appeared to be median proved to belong to the 

 right side and to correspond to the anterior pouch on the Kft. 

 Behind these on the right side came an unpaired iie[jhridium, 

 followed by a secon 1 normal pair. 



The question then arose, In what relations did the five 

 nephridia of this specimen stand to the normal four? Was 

 the unpaired pouch interpolated l)etween the normal two 

 pairs, or did it represent the second normal |)air, the fourth 

 and fifth pouches being an additional pair? It was obviously 

 impossible to obtain any direct proof on this point; but in 

 order to obtain some indication I measured the distances 

 between the successive nephridiopores in the abnormal and 

 in several normal specimens, at the same time taking the 

 distance between the genital hooks and the first pair of 

 nephridioj)ores as a standard of comparison. In the abnormal 

 specimen the distances were as follows : — (a) Genital hooks 

 to first pair of pores "5 millim. [h] First pair to unpaired 

 •25 millim. (c) First pair to second pair "7 millim. The 

 relation of distance (a) to distance {b) is thus 2 : 1, while that 

 of (a) to (c) is 5 : 7. 



in the normal specimens the relation of the averages of the 

 distances were : — Distance of genital hooks from first pair of 

 pores to distance between two pairs as 2 to 3, i. e. "5 to •75. 



•25 



(X=gen'tal hooks; O = uephridiopores.) 



