REVISE THE GROUP AND TO DETERMINE ITS GEOGRAPHICAL RANGE. 339 



(IV.) The po>terior ring or rings of bristles found in the genus Echiurus is of 

 great systematic value, but in counting the bristles it must not be forgotten on the 

 one hand that they often fall out, and ou the other it seems probable that the re- 

 placing bristle is sometimes counted as well as the one it is to replace. 



(V.i The longitudinal muscles. These are especially valuable in the genus Thalas- 

 sema, and serve to readily divide it into tw a - - with a continuous sheath 



of longitudinal muscles, and those with the sheath broken up into bundles. Some 

 species have beeu practically founded on the number of such bundles. As a rule the 

 number is small, i.e. below twenty. In using this as a criterion of species it must 

 not be forgotten that the muscles sometimes fuse and anastomose, thus diminishing 

 or increasing the number at any one level. I think it probable, but have no proof, 

 that they also increase in number with advancing age. 



\ 1 i The number of nephridia. These vary from a single one on the right side or 

 on the left, up to three pairs. They are perhaps - valuable aids to specific 



identification that the animals pn - 



(VII.i The anal trees. The variations in size presented by these organs are due 

 for the most part to th' si if contraction or expansion in which they were found 

 at the moment of death and are of little specific importance. Of greater value is the 

 primary or secondary branching of the diverticula which bear the funnels. 



In the following account I have arranged the struct' - - :es under the 



above headings and in the above order, adding here and there any other details which 

 seem to help in identifying the species in question. As far as may be I have given 

 the colour of the living animal, but the colour of the animal, live or dead, is sometimes 

 omitted in the original descriptions, and more often it is not mentioned whether the 

 colour is natural or the result of reagents. For myself whenever I see "yellowish- 

 brown," "greyish-yellow," and such sober hues. 1 suspect spirit. 



Until the ideal arrangement of concentrating all the type specimens of a group 

 into one Museum is attained, it is impossible to form a very definite opinion as to the 

 value of many species, and therefore in the following list I have practically included 

 all the specie> whose descriptions I could find. I am not however prepared to think 

 that they will all stand the I - time, and I am most doubtful about those whose 



claim to specific rank rests on one more or one less in the number of the longitudinal 

 muscle bundles, and about those described from single, sometimes fragmentary, specimens. 



The only description of a Thalassema which I have not incorporated in the following 

 lists is that of Th. verrucosa Studer 1 , from Betsy Cove. Kerguelen. It is too meagre 

 to permit of an opinion as to whether the species is new. or to tit into the key to the 

 species. The number of species of Bonellia and Echiurus is small, and they do not 

 require a key, but I have endeavoured to supply such an aid to identification in the 



1 Arch. Xr.tura., Jahrg. 45, 1879, p. 124. 



