216 M. E. Claparéde on a new Genus 
All these genera consist at present only of freshwater spe- 
cies. It is therefore interesting to make full acquaintance 
with a marine form, which certainly differs much from the 
types hitherto described, so that I have been obliged to form 
for it a new genus, which I will characterize further on under 
the name of Hemidasys*. 
Hemidasys agaso lives abundantly in the most muddy 
parts of the port of Naples; hence its specific name (agaso, 
groom). For a long time I regarded it as an epizoon. The 
surest means of procuring it is to examine carefully the speci- 
mens of Neretlepas caudata (Spio caudatus, Delle Chiaje). We 
soon meet with some individuals bearing one or two specimens 
of Hemidasys: these are fixed by their posterior extremity be- 
tween the feet of the Annelide. Their body, which is very 
contractile, elongates and contracts alternately, the anterior 
extremity feeling rapidly about in all directions, to seek its 
nourishment among the sete of the Neredlepas. Their move- 
ments resemble those of many of the Rotatoria. In attentively 
examining the mud, we find several free Hemidasyes; their 
being parasites, therefore, is only occasional or accidental. I 
have, however, never met with Hemidasys on other Annelides 
in the mud except Neredlepas. 
Hemidasys agaso attains a length of 0°3 to 0°5 millim., with 
an average breadth of 0°12 millim. Its form is that of a 
small band, or thick strap, with nearly parallel margins. In 
general it is more flattened than most of the other Gastro- 
tricha. The surface of the body is formed by a delicate 
cuticle separated from the adjacent parenchyma by a stratum 
of liquid of a shght rose-colour. The colour of this liquid is 
probably due to a simple effect of contrast, like that of the 
vacuoles and the contractile vesicles in the Infusoria. The 
liquid stratum is traversed by a great number of little bands, 
which pass directly from the parenchyma to the cuticle. At 
the point where it is attached to the latter, each band dilates a 
result. (‘Zur Kenntniss kleinster Lebensformen nach Bau, Function, 
Systematik, &c., von Dr. Maximilien Perty,’ Bern, 1852, p. 35.) 
* M. Mecznikow also mentions the genus Sacculus, ie On the 
other hand, the Russian naturalist does not mention Dasydites, Gosse, 
the diagnosis of which nevertheless dates back to 1851 (Ann. & Mag. 
Nat. Hist. Sept. 1851). In any case the genus Sacculus has nothing to do 
here. It was, it is true, classed originally among the Holotricha, Ehrb. ; 
but this not very natural order contains, besides some Gastrotricha, cer- 
tain true Rotatoria. The Sacculi have a mastax with two hammers and 
an incus. Their males are destitute of digestive apparatus ; in short, they 
are true Rotatoria in all points. M. Mecznikow certainly did not know 
them when he enumerated them among the Gastrotricha. Mr. Gosse, 
moreover, in his recent work on this group makes no mention of them. 
