38 MR. W. K. HUTTON ON THE [Jan. 20, 
investigated the Phascolosomide obtained by the ‘Travailleur’ 
and ‘Talisman,’ in speaking of classification, goes the length of 
saying, “‘ L’espéce bien affirmée n’existe pas dans le genre Phasco- 
losome”; and by this somewhat strong assertion he would 
indicate that the genus is in such a plastic condition—in a state 
of fluctuating variability as one may term it—that it is capable 
of being influenced by what Semper called ‘“ the conditions of 
existence” in such a way, that within the genus we may have 
numbers of types, differing on the whole but little from one 
another, the links of their genetic affinity easily supplied, which 
yet point the way toward the probable origin of the other 
Sipunculid genera. As Roule expressed it: ‘“‘ Ce genre est encore 
en voie de transformation, il se morcelle en un grand nombre de 
types peu dissemblables, et ne se divise point en espéces précises, 
dont les variations s’enserrent dans des limites peu éloignées.” 
Roule’s investigations led him to the conclusion that the Phasco- 
losomidee examined by him formed a series of links connecting 
the parent Phascolosoma stock with Aspidosiphon. In the phylo- 
genetic table drawn up provisionally by Selenka in his beautiful 
monograph ‘ Die Sipunculiden’ (page 6), a polyphyletic origin is 
suggested for both Phascolion and Aspidosiphon. The former 
may have arisen directly from the parent Phascolosomid, or from 
a common stem (similarly derived) which branches into Onchne- 
soma, Tylosoma, and Phascolion; while Aspidosiphon may have 
been derived from the directly produced Phascolion, or froma stem 
common to itself, Phymosoma, and Cloeosiphon. 
In Phascolosoma teres we have, as it appears to me, not perhaps 
an actually intermediate type, but at the least a species which 
gives us hints as to the probable path taken in its phylogenetic 
course by Phascolion ; while the species examined by Roule play 
the same role as.regards the genus Aspidosiphon. Phascolosoma 
sanderi, though considered by Collin to approach Golfingia, has a 
distinct but superficial resemblance to <Aspidosiphon, and was 
rightly, as I think, placed by its describer in the genus Phascolo- 
soma. 
In the worm which forms the subject of this paper the follow- 
ing characters suggest Phascolion-like aftinities:—The differing 
bulk of the two nephridia, the relatively feeble development of 
the dorsal retractors (a character upon which Roule placed much 
reliance in his investigations), the lack of spindle-muscle, and the 
coiled, but not spirally-wound gut,—all these are peculiarities 
which place the worm in closer relationship with Phascolion than 
‘with any other of the groups derived from the original Phasco- 
losomid stock. 
Information as to the habits of Phascolosoma teres is wanting ; 
but in its mode of life it is probably sluggish, lying buried in soft 
ooze with its anterior end alone projecting. The thin-walled 
bag-like posterior portion, endowed with but feeble muscularity, 
so sharply demarcated from the rest of the body, so relatively 
large, would undoubtedly act as a hindranee to active locomotion 
