THE DEGENERATION OF ARMOUR IN ANIMALS 129 



pelagic), the Pteropoda, on the other hand, mark the present 

 limit of evolution from the same group in the direction of a 

 free-swimming, pelagic existence. Here again, as in the sea 

 slugs, a double origin for this group has been suggested : the 

 Thecosomata, in which a shell is present in the adult, being 

 derived from the more primitive group, the bubble sheik or 

 Bulloidea; the naked, carnivorous Pteropoda Gymnosomata 

 (with only a larval shell) from the more advanced section, the 

 sea-hares or Aplysioidea. It is, however, only in the family 

 of Limacinidce that the shell has retained its spiral form (e.g. 

 Spinalis and Limacina, figs. 15 and 16); in the Cymbulidce it is 

 certainly a new structure altogether ; in all probability this is 

 also the case with the Cavoliniidce (fig. 17), the third remaining 

 family of the Thecosomata. 



A somewhat analogous, although less well-established case 

 of the degeneration of the shell in Gastropoda, concomitant 

 with increased activity of the individual organisms and wide- 

 spread distribution of the family, is to be found in the Strepto- 

 neura. Although it is somewhat hazardous to deduce affinities 

 from fossil shells of which the animals are completely unknown, 

 many authorities concur in regarding the thoroughly pelagic 

 class of Heteropoda as derived from the dominant Palaeozoic 

 family of the Bellerophontidcc, in spite of the fact that inter- 

 mediate forms are hardly known ; but want of continuity is 

 probably due more to lacunae in the present state of our know- 

 ledge, which may at any time be filled up by some fortunate 

 find (as in the case of Archccopteryx). Yet Bellerophon shows 

 a progressive widening of the body whorl, which (as already 

 seen in the Bulloidea) is associated with an increased develop- 

 ment of the body and a corresponding degeneration of the 

 shell. Some of the Silurian forms {Salpingostoma, Tremanotus) 

 are much more expanded and possess wider mouths than the 

 Ordovician representatives {Cyrtolites omatus, Bellerophon argo, 

 fig. 18, B. acutus) of the family; this tendency is still notice- 

 able in the Carboniferous forms — e.g. Bellerophon bicarenns 

 (fig. 19), B. costatus. In point of massiveness of shell, Bellerophon 

 can compare with Actcvonclla, whilst the modern Heteropoda 

 have as thin and delicate a shell as the equally pelagic 

 Pteropoda. The small, globular Bellerophina minuta (fig. 20) 

 of the Gault may perhaps be regarded as belonging to the 

 Heteropoda; and the shell of the modern Atlantidce, which still 



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