chap. XII.] COLOURS OF CHITOXS. 307 



remarkable that though there are no spiracular openings on the meso- 

 and meta-thoracic segments, yet in cases of extremely spotted larvae 

 there are red spots at the level of and continuing the spiracular series 

 of spots upon these segments also (S. ocellatus, Buckler, PI. xx. 

 tig. 1 a; Poulton, 1887, PL x. fig. 1. S. populi, Poulton, 1887, p. 286). 

 As an indication of an element of definiteness in this variation may 

 be mentioned the fact that in fully spotted larvae of S. populi the 

 sub-dorsal spot on the 7th abdominal seems to be always the smallest 

 in that row (Poqlton, 1887, p. 285; Wilson, PI. v. fig. 2a; Flemyng, 

 Ent, 1880, p. 243, &c). 



In our present consideration the fact that these very large variations 

 sometimes occur simultaneously over a large range of segments and 

 are sometimes restricted to particular segments is of considerable 

 importance. 



We may note that Weismann (p. 360) is prepared to believe that 

 these spots represent a new variation arising similarly and indepen- 

 dently in the different species of Smerinthus. As however is usual in 

 cases of considerable Variation an attempt has been made to lessen 

 the value of these indications of the magnitude of Variation by sug- 

 gesting that they may be of the nature of " reversion " (Poulton, 

 1884, p. 28). Apart however from a general reluctance to recognize 

 the possibility of the occurrence of large variations there seem to be 

 no special grounds for the suggestion here. It is nevertheless true 

 that in the case of the Smerinthus larvoe a complete disproof of the 

 hypothesis of "reversion" is wanting. This is only to be obtained in 

 cases (like that of D. euphorbice), in which a great number of complex 

 and mutually exclusive variations exist side by side. In the absence 

 of such complete refutation the hypothesis of reversion may still find 

 favour. 



'471. Chitonidae. The following facts observed in certain Chitons 

 are given in illustration of the existence of a similar possibility 

 of simultaneous Variation between parts which are repeated in 

 series but whose repetition is not of the kind commonly included 

 in the term Metameric. Unfortunately the material at hand is 

 very limited and I do not know what might be the result of 

 further examination, but the facts seen suggest that the subject 

 is worth investigating. 



The dorsal plates of Chitons are eight in number. Though 

 the colours and markings in different species are complex and 

 various yet in many species all the plates are alike or nearly so. 

 The question then arises do all the plates change colour together, 

 or do they change one by one, or otherwise ? From the few 

 observations made it seems that in this respect the species differ, 

 but variation uniformly occurring in all the plates seems to be 

 rare. This may perhaps be due to the constitution of such 

 specimens as separate species, but I saw little likelihood of this. 

 On the other hand in several cases the same variation was present in 

 more than one segment, and in particular there was strong evidence 

 that in some species the segments 2, 4 and 7 shew a noticeable 



20—2 



