40 THE ANCESTRY OF VERTEBRATES 



shorter canals, as in the Annelids Arenicola marina, Bran- 

 chiomma vesiculosum and others, in the primitive Lamellibran- 

 chiata Nucula, Leda, Malletia, Solemya, where the canal opens 

 at both sides of the surface of the foot, and among Vertebrates 

 in the Elasmobranchs where the two ductus endolymphatici 

 open dorsally. In other cases we find a blindly ending 

 canal as a last remnant of such a communication, like the 

 KoLLlKER's canal in Cephalopods and the ductus endolym- 

 phaticus in most Vertebrates^ where it fails only in Teleos- 

 teans. Just as in the case of the eyes it is in Cephalopods 

 that the organ of equilibrium, lastly described by HARRIS ( 1 903), 

 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 {he 

 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 

 KoLLlKER'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 Hatsghek (1886, plale 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 



