226 
shorter canals, as in the Annelids Arenicola marina, Bran- 
chiomma vesiculosum and others, in the primitive Lamellibran- 
chiata Nucula, Leda, Malletia, Solernya, where the canal opens 
at both sides of the surface of the foot, and among Vertebrates 
in the Elasmobranchs where the two ductus endolymphatict 
open dorsally. In other cases we find a blindly ending 
canal as a last remnant of such a communication, like the 
KÖLLIKER’s canal in Cephalopods and the ductus endolym- 
phaticus in most Vertebrates, where it fails only in Teleos- 
teans. Just as ín the case of the eyes it isin Cephalopods 
that the organ of equilibrium, lastly described by HARRIS (1903), 
exhibits the highest differentiation among Protostomia and 
at the same time, just as the eyes, a certain resemblance 
to the corresponding organs of Vertebrates. In Nautilus the 
statocysts are closely applied to the cartilaginous endo- 
skeleton, in the Dibranchia they are wholly embedded in it 
so that we can distinguish here a membranous and a 
bony (cartilaginous) labyrinth. Besides the blindly ending 
KÖLLIKER’s canal, the wall of the statocyst is raised into 
several well-marked ridges separated by furrows. The sensory 
epithelium is restricted to one macula acustica, or, better, 
macula statica princeps (HARRIS, l.c.p. 330) on which the 
large statolith rests, to two maculae staticae neglectae 
(at least in Decapods), on which a great number of little 
statoconia, embedded in a gelatinous mass, are found, and 
to a crista statica. The nerve supplying the organ sends a 
branch to the maculae and to the crista. 
In Annelids the statocysts are neither so common nor 
such typical organs as in Molluscs but in their situation and 
their structure they exhibit in both groups a great resemblance. 
The last author who has studied the statocysts in worms is 
FAUVEL (1902, 1907). They are found only in four families, 
all sedentary, viz: the Ariciidae, Arenicolidae, Terebellidae 
and Sabellidae, of which 34 species with 'statocysts are 
enumerated. In Molluscs, on the contrary, they are of extremely 
general occurrence and even in a few Polyclads and 
Nemerteans paired statocysts are found. 
In Molluscs as well as in Annelids the statocysts appear 
very early in ontogeny, as a rule already in the trochophora 
in such forms that have one. For Annelids this is shown 
e.g. by HATSCHEK (1886, plate V). They take their origin 
from the lateral body wall between mouth and anus, as 
a rule not far behind the mouth. Often soon after their 
