288 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



( Trachydermon) cinereus is from Ihering. It will be seen that there are 

 comparatively few important differences between the twoj the pedal 

 commissures (wpc)\ the separation of the ganglia App from close con- 

 nection with the anterior (iapc) and posterior (pipe) loops; the larger 

 and more conspicuous buccal ganglia (8p) and the less complete coales- 

 cence of the strands forming the pedo-branchial commissure (PBC) are 

 the most conspicuous features. Further research is required to deter- 

 mine how much of these differences is.due to the diagrammatic character 

 of the figures, and how much to the systematic difference between Tra- 

 chydermon and Acantliochiton. 



The nervous system of Acanthochiton fascicularis chiefly consists of 

 two large angular ganglia bound together by a large flat commissure. 

 These two principal ganglia, which lie on the sides of the buccal mass, 

 may be taken as a consolidation of the ganglia pedalia and the ganglia 

 branch ialia sen visceralia ; thence springs out a nervus pedalis, which 

 supplies the foot and muscles with minute rami on each side of the 

 nerve; also a nervus branchialis, which passes along a furrow on the 

 inner edge of the mantle, giving out secondary rami to the branchia?. 

 The cerebral ganglia are wanting, unless we consider with Middendorf 

 that they form part of the pedo-branchial commissure. Brandt objects 

 to this view on the ground that the commissure throughout its whole 

 breadth is similarly formed and gives out similar nerves; namely, nervi 

 labiales from in front, and a multitude of minute nerves to the pharynx 

 behind. As Chitons have in the adult condition neither eyes nor tenta- 

 cles, so the absence of these ganglia (from which in other forms nerves 

 are given out to those two organs only) seems very natural. This com- 

 missure may also be called the pedo-branchialiSj and it may correspond 

 with the commissura cerebralis, from which similar nerves have been 

 demonstrated to spring. This commissure also presents resemblances 

 to the nerves and ganglia of the stomato-gastric system, common to 

 many gasteropods, in its intense yellow color. A commissure binds each 

 pedo-branchial ganglion with a little inferior pharyngial ganglion, and 

 the same also connects these inferior pharyngial ganglia with one 

 another by an inferior interpharyngial commissure (as in Patella vulgata, 



lures, generally more characteristic of Annirfosa, from the different times at which 

 they started from the common stock on an independent career of specialization. 



All this in no wise authorizes the combination in one group of worm-liko mollusks 

 and molluscoid worms. The writer has persistently opposed such ill-considered con- 

 glomerations as wholly unphilosophical. Even were there embryological identity, 

 which no one has claimed, such a course seems to him to indicate an ignorance of tho 

 meaning of terms in systematic nomenclature, or the confounding of the two starting 

 points for classification, to which allusion has been made. He will oven venture to 

 predict that when the anatomy and developement of two hundred, instead of two, 

 species of Chitons and Limpets, are worked out, a single phylum will express their 

 relations to the worms, to each other, and to the other true gasteropods; and to assert 

 that, in his opinion, nothing is so likely to conduce to this simplification than the 

 continuation and amplification of the really admirable work upon which Dr. v. Ihering 

 and others have of late been engaged. 



