BCRNE : AjSTATOMY of nANLEYA ABTSSOEUM. 1 .i 



a single posterior unpaired opening ; and finally, in one case (a species 

 allied to C. 'monticularis, Quoy) a single pair only. The rarity of the 

 final condition, in which only one pair is left, makes it a noteworthy 

 fact that this type is also to be found in Ilanleya ahyssorum. In this 

 Chiton the ventricle is short and rounded (more so than in Haller's 

 figure, but the shape may be a good deal dependent upon the con- 

 traction caused by the preserving fluid) ; its posterior extremity rests 

 loosely upon the united auricles, but is in no way attached to them ; 

 the single pair of auriculo-ventricular openings are situated rather 

 towards the anterior end of the ventricle. 



The above deviations from the structure of the more commonly 

 observed species of Chiton, most of them tending in the direction of 

 concentration, and suggestive of a more specialized condition — e.g. 

 separate olfactory ganglia, enlarged anterior and posterior pedal 

 commissures (the only two still persistent in Patella '), the concen- 

 tration of the lateral cords in the branchial region, and the condition 

 of the heart — gain a new interest when considered in relation to the 

 habitat of their possessor ; for we find them congregated in a creature 

 that has strayed from the haunts of its surf-loving relatives into the 

 home of the more specialized Aplacophora, a region verging upon the 

 deep sea.'^ 



My best thanks are due to my friend and former teacher. Professor 

 G. B. Howes, for his kindness in reading over and criticizing this 

 paper, and to Professor C. Stewart for permitting me to examine the 

 gills of some of the rarer species of Chiton. 



EXPLANATION OF PLATE II. 



Fig. 1. — Model of the tenth gill from the left side of Hanleya ahi/ssorum, recou- 



structed from sections. x 30. a.v. main afferent branchial vessel. 



av'. afferent vessel of the individual gill. e.n. external branchial 



nerve, e.v. main efferent branchial vessel, e.v'. efferent vessel of the 



individual gill. i.n. internal branchial nerve, l.n. lateral nerve-cord. 



m.n. mantle nerves. (/(/. proximal ganglion on the external branchial 



nerve. 

 Fig. 2. — Transverse section of a portion of the gill of Haideya ahi/ssor/im. x 80. 



e n. external branchial nerve, ep. epithelium, e.v. efferent vessel. 



ff.l. gill lamella. 

 Fig. 3. — Transverse section of a portion of the gill of Acanthochites discrepans. 



X 240. e.n. external branchial nerve, e.v. efferent vessel, p.ep. 



pigmented epithelium. 



These figures were drawn Avith the camera lucida. Fig. 3 is a combination of two 

 sections, in one of which the pigment cells were more distinct, in the other the 

 hair cells. 



> E. L. Bouvier, " Systeme nerveux des Gasteropodes Prosohranches " : Ann. 

 Sci. nat. (ZooL), ser. vii, iii and iv, 1887, p. 17. 



^ H. Simroth, " Kritisehe Bemerkungen iiber die Systematik der Neomeuiden" : 

 Zeitschr. "\Yiss. ZooL, hi, 1893, p. 326. 



