. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 260 L. R. McEDWARD AND D. A. JANIES "; We believe that such a definition is impossible for two reasons: (1) the continuous nature of development, and (2) the evolutionary diversification of development. First, "larva" refers to a stage (or series of stages) in a developmental sequence. In asteroids, development does not progress as a sequence of discrete instars but rather involves continuous changes in morphology. Stages can- not be precisely defined, because they do not begin and end with unambigu
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 260 L. R. McEDWARD AND D. A. JANIES "; We believe that such a definition is impossible for two reasons: (1) the continuous nature of development, and (2) the evolutionary diversification of development. First, "larva" refers to a stage (or series of stages) in a developmental sequence. In asteroids, development does not progress as a sequence of discrete instars but rather involves continuous changes in morphology. Stages can- not be precisely defined, because they do not begin and end with unambiguously identifiable developmental events. The larval form is produced by morphogenetic processes that gradually transform the embryo into a more complex shape. Larval stages transform into juvenile stages during metamorphosis, which can be rapid and drastic or prolonged and gradual. Second, developmental sequences may have been greatly modified (accelerated, retarded, and condensed), and the stages within those se- quences may have undergone evolutionary modification. Given the tremendous morphological, ecological, and taxonomic diversity of marine invertebrates, it is not sur- prising that a single definition fails to be universally ap- plicable. For evolutionary studies, we maintain that a larva should be defined as an intermediate stage in the life cycle that is produced by post-embryonic morphogenesis and is eliminated by the metamorphic transition to the ju- venile; in addition, this intermediate stage must possess transitory structural features that are not developmentally necessary for morphogenesis of the juvenile. Transitory larval features may be specialized for independent exis- tence during development, or be reduced in derived modes of development. Juvenile The term "juvenile" has been used in a very broad sense. At one extreme, it refers to the "post-larval" stages, either immediately following abrupt metamorphosis from a definitive larval stage
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
