TITANOSOMA AND POLYMICRODON IN ENGLAND 1 53 



only a single female at my disposal, I should like to add the 

 following particulars. My previous supposition that Titano- 

 soma reaches a stage with twenty abdominal segments, and 

 that therefore my type specimen represented the last larval 

 stage, appears very improbable in view of the fact that I now 

 have numerous examples with nineteen segments, but none at 

 all with twenty. The fact that I detected no vulvae might be 

 partly due to their small size, partly to the parthenogenetic 

 reproduction. 



If adult TitanosomcB have nineteen abdominal segments, we 

 must regard Brolemann's Macrosternodesmus as the most 

 nearly related genus. As characterising this genus in contra- 

 distinction to Titanosoma the following particulars are all that 

 can be brought forward for the time being : 



"Tous les teguments presentent une structure reticulee, 

 due, semble-t-il, a de minuscules vermes plates, regulierement 

 distributes sur toute la surface ; sur les prozonites ces vermes 

 sont orientees longitudinalement, de facon a former des 

 rangees transversales." " Somite preanal long, en forme de 

 grand crochet rapidement aminci et courbe vers le sol; sa 

 pointe tronquee depasse le niveau du bord des valves anales, 

 qui sont cependant tres proeminentes." 



In Macrosternodesmus it therefore appears that the pleuro- 

 tergites, and especially the prozonites, possess a much stouter 

 structure than in Titanosoma, since this form shows dorsal 

 plates which are for the most part smooth ; only under strong 

 magnification (about two hundred and twenty diameters) is it 

 possible to see here and there a fine reticulation, which is 

 most clearly recognisable as a wavy cross-furrowing in front 

 of the transverse constriction. In the neighbourhood of the 

 pleurae, and especially above the stigmata, the reticulation is 

 much plainer. The anal segment of Titanosoma is un- 

 questionably different from that of Macrosternodesmus 

 palicolus. In side view the process of the telson appears 

 short and rounded, somewhat thickened at the end, and not 

 bent towards the underside. On the dorsal side of the telson 



