94 Mr. E. Kaj Lankester on tlie Organization 



this species, consequent on the observation of the hist sjDecies, 

 has shown me that in T.rivulorn.m there is a rudinientar j web 

 to the dorsal sette of the first ten fasciculate segments, and 

 even traces of such a web as far as the fifteenth. This and 

 the peculiar form of these set^ has not before been described. 

 Though considerably smaller relatively, yet the setai in this 

 region approach those of T. umhelUfer also in form, having 

 the form seen in Z>, J', fig. 2, with the prongs nearly straight, 

 unlike those of the ventral region and of the other parts of the 

 body (a, fig. 2). It requires a glass of very good power to sec 

 this web well — a yg- or Hartnack's 10 a immersion. It is 

 most developed in the set^ of the sixth and seventh fascicles, 

 and is to be made out, though very slight, in the setffi of the 

 fourteenth fascicle, where the prongs have assumed the unci- 

 nate form characteristic of most Oligocluetous crochet-bristles. 

 A remarkable fact is, that in young (/. e. small) specimens of 

 T. rivnlorum the webbing of the bifurcation of these bristles 

 is more obvious tlian in the older and larger individuals. 

 This suggests the supposition that T. umhelUfer represents a 

 more primitive form, and that the rudimentary webbing 

 of the setaj of T. rivuJoriim is a case of retention, in a rudi- 

 mentary state, of ancestral characters which were formerly 

 highly developed. When it is remarked, further, that such a' 

 form of seta is unknown except in marine Annelids, and that, 

 as far as it appears, T. umheUifer is a brack ish-wafer form, the 

 rudimentary webbing in T. rivnlorum becomes more important. 



I have seen no trace of such webbing in the setae of Limno- 

 drilus (which is consequently thus further separated from 

 Tubifex), nor in any other Oligochgetous Annelid examined 

 with care for this purpose. 



Four or five very fine hairs, of six times the length of the 

 sette, are often to be seen, in small specimens of TuhifeXy sur- 

 rounding each seta near its apex ; they apparently result from 

 the splitting-up of the horny substance of the seta ; and they 

 occur in Nais as well. Small dark particles are placed at in- 

 tervals along these fine hairs. These appearances are proba- 

 bly pathological, but are so common as sometimes to lead one 

 to suppose them characteristic and normal. 



4. Enchytro'us and Pachydrilus. — In a garden-heap I have 

 obtained specimens of the typical Enchytrams vermiciilaris ; 

 whilst from a pond at Hampstead, from a running stream, 

 and from the Victoria Docks I have obtained worms which 

 should be referred to EnchytrcEus^ but possess blood coloured 

 red by luemoglobin. That from the Hampstead pond is 

 marked with light-yellow^ bands externally, and is otherwise 

 colourless. There does not seem to be sufficient ground for 



