46 . THE VOYAGE OF H.M.S. CHALLENGER. 



Chitons it must certainly be looked upon as the more primitive and the more closely 

 related to the Solenogastres." 1 



On the other hand, Dall finds "in Chitonellus fasciatus the representative of the 

 most highly developed type of Chiton." 2 



The shell is remarkable in many ways, not least for the great development of the 

 articulamentum as compared with the tegmentum, and, with the exception of three slits in 

 the anterior valve, all the insertion plates are unslit. 



The structure of the shell indicates that the Leptochitons are the most primitive forms. 

 In the group of the Leptoidea, the sutural laminae are small and are present in all the 

 valves except the first, but the laminse of insertion first appear on the terminal valves 

 only, next on the intermediate valves also, in all cases they are small and never slit. In 

 all other Chitons the insertion plate of the anterior valve is slit, but in Cryptoplax alone 

 the lateral insertion plate is unslit. Cryptoplax agrees with some of the Acanthoidea and 

 with the Schizoidea and Placiphoroidea in having the insertion plate of the terminal 

 valve also unslit. It is probable that this condition has been independently accpiired in 

 several of these instances, whereas in Cryptoplax it is possibly a primitive condition. 



The scales of the girdle in the Leptoidea are delicate and scattered, and in the higher 

 members of the series (Hanleyia and Hemiarthrum) small sutural tufts appear, and in 

 the latter, at all events, there are anterior tufts as well. The presence of sutural tufts 

 appears to be quite irregular, even in closely allied forms ; as, for instance, among the 

 subgenera of Plaxiphora and in the genus Cryptoplax itself. We may conclude then 

 that the possession of tufts is not of very great importance, at all events when they are 

 feebly developed, and especially when they have a tendency to disappear. It must not be 

 forgotten that there are amongst the Ischnoidea forms with and without girdle pores ; in 

 Angasia there are stated to be sutural hair-tufts. It may be that, with the exception of 

 the Leptoidea, the pore-bearing forms represent the lower members of their series. 



The microscopical structure of the spines of the girdle of Cryptoplax (Cryptoplax 

 larvceformis and Cryptoplax striatus) is identical with that of Acanthochiton (Acantho- 

 chiton discrepans, Brown, and Acanthochiton gamoti, Blainv.), and differs from that of 

 all the other species of Chitons which I have as yet examined. These spines may fairly 

 rank as the most specialised in the whole group. 



The great thickness of the girdle itself in Cryptoplax certainly recalls the thick spini- 

 ferous body wall of the shell-less Isopleura, and the comparatively narrow foot lying within 

 the pallial groove bears but slight resemblance to the incipient foot of Chcetoderma or 

 Proneomenia. 



The gills present us with some points of interest. In Proneomenia amongst the . 

 shell-less forms gills are entirely absent, respiration, according to Hubreeht, probably taking 

 place all along the wall of the intestine and the foot, and perhaps more especially in the 

 1 Quart. Journ. Mie. Scl, 1882, p. 214. 2 Proc. U.S. Nat. Mus., 1878, p. 314. 



