Prof. E. Claparede on the Structure of the Annelida. 343 



A second inconvenience of the nomenclature of M. de Quatre- 

 fages is that it is inapplicable in all those cases in which the 

 anterior segments are much condensed, and in which it is no 

 longer possible to determine to what segment a given pair of 

 appendages belongs. We shall see, for example, that in the 

 Phyllodocea and the Hesionea authors are unable to agree upon 

 this determination, and that M. de Quatrefages allows himself 

 to be led away by his theory of the appendages to establish 

 genera which no one will adopt. We also find the learned 

 Academician, for love of his theory, suppressing by a stroke of 

 his pen the buccal segment in most of the Sigalionida, or at 

 least attributing to them "an indistinct buccal segment, desti- 

 tute of appendages." But nothing is more distinct than the 

 buccal segment of these Annelida ; only it bears a pair of feet 

 with setse, which a buccal segment ought never to do, ac- 

 cording to the theory of M. de Quatrefages. Unfortunately the 

 author does not suspect that all the Polynoa likewise bear some 

 setae on the segment which he regards as the buccal ring, and 

 that it would consequently be necessary to imagine in them an 

 " indistinct buccal segment without appendages." 



M. de Quatrefages, however, gives us a rule (difficult of appli- 

 cation indeed, but still a rule) for the determination of the seg- 

 ments and their appendages. The cephalic lobe and the an- 

 tennae, he says, I'eceive their nerves from the cerebral ganglion, 

 the buccal segment and its tentacles from the oesophageal con- 

 nectives, and the tentacular cirri from the ventral ganglionic 

 chain. This thesis is not tenable in presence of the modern 

 progress of embryology. Schaum asserted that in all Arti- 

 culata a segment is characterized by the presence of a ganglion, 

 and he started from this principle in denying that the head 

 in Arthropoda is formed of several segments amalgamated to- 

 gether. This doctrine was immediately refuted. In fact, the 

 nervous system is comparativelj; -"ry late in being differentiated 

 in the embryos of Articulata ; on the contrary, the appearance 

 of the segments (the protoconites as they have been called) is in 

 many cases the result of one of the first modifications of the 

 blastoderm. These primitive segments unite in groups, and 

 sometimes become soldered together, long before the differentia- 

 tion of the nervous system ; and when this system is developed, 

 the number of its ganglia is not necessarily identical with that 

 of the primitive segments. In the Annelida especially, the 

 formation of the nervous system certainly sometimes follows 

 very closely upon that of the segments, as in the embryos of 

 Capitellus, for example ; but most frequently it is much later. 

 I do not, indeed, dispute that in many Annelida the origin and 

 distribution of the nerves is in accordance with the rule of M. 



