810 



VARIATION AND REGENERATION. 



like the latter they are laid singly. The pectinate ridges are more prominent on the 

 outer capsule of the egg of N. pompilius, often having a frill-like appearance and pro- 

 jecting freely for a short distance at their anterior ends. 



The capsules of the egg are often deficient in some respects, sometimes exposing 

 the vitellus through the gaping sutures of the 

 inner capsule (PI. LXXXIII. fig. 18), and in 

 the case of N. macroinphalus at Lifu I frequently 

 found fully formed capsules without any con- 

 tents, the latter not having escaped, but never 

 having been present. This also must I think 

 have been due to a derangement of the normal 

 reflexes, the result of capti\ity. The pectinate 

 ridges of the outer capsule of the egg seem 

 to correspond with the annulations of the infra- 

 buccal tentacles (PI. LXXIX. figg. 2 a and 6 a). 



For further details concerning the egg- 

 capsules I may refer to the explanation of 

 figures 19—23 on Plate LXXXIII. 



Fig. 14. Three eggs of N. macromphalus laid in 

 a row, with capsules coalcscent. 



21. Vaei.\tion and Regexeratiox. 



Although the old idea of the fixity of species has long been superseded, the fixity 

 of certain tj-pes seems to be a fact not open to question. Fixity of tv'pe is the ex- 

 pression of a completed evolution, and this is what is meant when a particular t\-pe 

 is said to be highly specialised in a given direction. Fixed t\-pes are verj' likely the 

 same as persistent types, and they have some properties in common amongst which are 

 their limited range of variation and their defective power of regeneration. 



The variation in the number and disposition of the digital tentacles of Nautilus 

 is a negligible quantity, but the spadix exhibits ambidextral variation, being now on 

 the left side of the cephalopodium and now on the right. A variation of a similar 

 nature affects the origin of the main siphuncular arter}-, which sometimes arises as a 

 branch of the left division of the posterior pallial artery and sometimes from the right 

 division. The constitution of the spadix shows hardly any appreciable variation. 



In one instance only did I meet with a situs inversus of the reproductive apparatus, 

 and this was the case with a male in which the vas deferens was developed on the 

 left side instead of on the right, wth the pyriform gland on the right instead of the 

 left side (PI. LXXVIII. fig. 2). 



More than once or twice I found males in which the hood had been so bitten away 

 that the spermatophore lodged in the buccal recess was exposed to view. Sometimes 

 the front part of the hood is eaten away, at other times a piece is bitten out of 

 the middle of the hood just over the spermatophore (PI. LXXVII. figg. 5 and 6). 



At first I attributed these injuries to the attacks of voracious fishes, but it is not 

 impossible that they are the results of nuptial combats. Without in any way relaxing 



