By GFosta JFagersten
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The learn of the historical past of lifestyles encompasses the origins of species to their loss of life: evolution and extinction. in response to experiences of the fossil checklist, this ebook examines the directionality of evolution--so known as evolutionary trends--a basic point of lifestyles historical past that has obtained relatively little recognition in recent years.
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Extra resources for Evolution of the Metazoan life cycle: a comprehensive theory
The approximate position of their evolutionary emergence is marked in the phylogenetic diagram (Fig. 1 1 ). It is of interest that recent brachiopod larvae, particularly distinct within the Testicardines (Fig. 8), exhibit both kinds of mantle structures. In the foregoing treatment of the mantle-shell complex I have pointed to the close morphological agreement with the corresponding structures in Mollusca, as does Lemche ( 1 963). But unlike this author I am unable to interpret the agreement otherwise than merely as a phenomenon of con vergence.
I consider it improbable that thin, delicate mantle lobes should have been formed without connection with overlying shells. I want to stress that this reasoning refers to the adult. When the onto genetic emergence of mantle and shells was shifted to the pelagic phase of the life cycle, a larva resulted which, although perhaps not identical with a cyphonautes in every detail, agreed with it in essential points. ) Compara tively heavy shells seem, however, rather unsuitable for a pelagic larva (cf.
Lang has been able to establish that at the time of hatching Priapulus has no armour, but within two days a thin hyaline layer can already be seen over the cells of the ectoderm. He believes this layer to represent the beginning of the armour, which thus is apparently formed at a very early stage. If we disregard the armour and, as far as Priapulus is concerned, the small " foot " at the posterior end (see below) there is on the whole close agreement between larva and adult. In my opinion there can be no doubt that there has been a change from the original type of ontogeny with a pelagic primary larva to holobenthic direct development and that after wards a secondary divergence between juvenile form and adult has taken place, resulting in the present secondary larva.