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المرحلة 1
أستاذ المادة محمد سطام عباس التميمي
4/19/2011 8:04:11 PM
Phylum Brachiopoda
6.1 Introduction
While recent brachiopods are a rather rare and insignificant group, their long fossil history shows that they were at times the most prominent animals in the seas. Consequently, brachiopods receive only passing interest from zoologists, but a great deal of attention from paleontologists. The phylum is quite important for biostratigraphy, paleoecology, and evolutionary studies because it shows a great variety of changes in form and function through time.
Brachiopods appear near the beginning of the Cambrian, but did not become abundant until the Early Ordovician. The remainder of the Paleozoic could be termed the Age of Brachiopods-several orders dominated the shallow shelf environments throughout the era, giving way only reluctantly to the rapidly diversifying bivalved molluscs and gastropods of the Mesozoic. Only a few groups survive today.
6.1.1 Functional Morphology
General Form
Brachiopods are solitary, entirely marine animals, each with a shell consisting of two opposing parts (valves) that enclose most of the soft body. The animal and its shell are bilaterally symmetrical about a plane drawn perpendicular to the line of contact of the closed valves (the commissure). In most brachiopods, the shell is made of calcite, but a few groups have shells made of calcium phosphate with varying amounts of organic material.
Figure 6.1 Cross section of a brachiopod showing the relationship of the internal organs to the two valves (from Stearn, 1989).
Feeding
Inside the shell is the feeding structure characteristic of the brachiopods-the lophophore. This consists of a pair of ciliated, twisted projections that create water currents and then filter out microscopic food particles. Often the lophophore has a calcareous supporting brachidium. Fossil and Recent brachiopods have a variety of accessory supports for the feeding apparatus.
In order to increase the amount of water filtered and still protect the delicate lophophore from overly large particles, some brachiopod lineages (notably the Rhynchonellida) developed a zigzag commissure.
The zig-zags bring the sensitive mantle edges closer together, giving the animal more control over the quality of incoming material (Figure 6.10c shows a zig-zag commissure).
Articulation and valve movement
The most common class of brachiopods, the Articulata, is characterized by the presence of two opposing calcareous valves hinged along the posterior edge. They usually have a series of sockets and teeth which allow valves to open anteriorly for feeding; they can also keep the valves firmly closed when necessary. In some brachiopods the articulating structures have been reduced or lost during evolution.
Two major muscle sets open and close the valves. Diductor muscles attach at one end to the floor of the ventral valve, and at the other end to a projection (cardinal process) in the dorsal valve. When these muscles contract, the hinge acts as a fulcrum, opening the valves anteriorly. Adductor muscles, which are attached between the floors of both valves, contract to close the valves and hold them shut.
Figure 6.2 Brachiopod valves are opened and closed by pairs of opposed muscles: (A) Contraction of adductors closes the valves; (B) Contraction of diductors levers the shell around the hinge and opens the dorsal valve (Boardman et al. 1987; from Prothero, 1998)
Relation to substrate
Most brachiopods have a fleshy stalk, termed the pedicle, that protrudes posteriorly through one valve or between the valves and attaches permanently to the substrate. When the pedicle exits through a valve (by definition the ventral valve), it leaves an opening that varies greatly in form among brachiopod groups.
In many the pedicle was lost during either ontogeny or the evolution of the lineage, leaving as evidence a hole partially or completely closed off by accessory plates or growth of the ventral valve.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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