. . sation ofgrowth in Apus before the full number of inheritedand rudimentary segments are fully developed. Apusis thus, even when adult, little more than a largeNauplius with its posterior segments in front of theanal segment fixed throughout life in their larval con-dition. The significance of this fact is very great,it shows so conclusively that Apus is a primitiveform, that we cannot refrain from repeating ourexplanation of its morphology. The very factwhich has been supposed to be an index of the highspecialisation of the A pod
. . sation ofgrowth in Apus before the full number of inheritedand rudimentary segments are fully developed. Apusis thus, even when adult, little more than a largeNauplius with its posterior segments in front of theanal segment fixed throughout life in their larval con-dition. The significance of this fact is very great,it shows so conclusively that Apus is a primitiveform, that we cannot refrain from repeating ourexplanation of its morphology. The very factwhich has been supposed to be an index of the highspecialisation of the A pod id re, , the great numberand peculiar arrangement of the limbs, is in realityone of the strongest proofs of the undiffcrentiated 166 THE PART I primitive character of the genus. The number oflimbs is far in excess of the rings in the body, and ifwe once recognise that the rings do not correspondwith segments except in the fully developed anteriortrunk region, but that each pair of limbs having its ownpair of ventral ganglia corresponds with a true segment. v FIG 29.—Second larval stage of Apus cancriformis (Claus), showing the gradualdevelopment of Apus out of the Nauplius, the liver as diverticulaof the , frontal sensory organs. either developed or rudimentary, we have an animal,say Apus cancriformis, with from 60-65 are other species with from 40-50 all other Crustacea the number of segments isfor each group either absolutely or very nearly con-stant. The type is fixed. In the Apodidre, as we SECT. XI DEVELOPMENT 167 liavc seen, this is not the case ; the number ofsegments varies not only in the different species ofthe genus, but, as it appears, in different individualsof the same species. These two characteristics of theApodidae, the great number and the varying numberof the segments, ought almost of itself to constitutethem the natural transition form between the Annelidsand the Crustacea. In the Annelids we have a largeand variabl
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