Physical characteristics

Adult kinorhynchs range in length from 0.008 in (0.2 mm) in certain Echinoderes, to 0.05 in (1.2 mm) in the arctic Pyc-nophyes. Most are transparent; but there are yellowish and reddish species that live on macroalgae. A few have red or brown cup-shaped eyes located in the head near the brain.

The body is subdivided into two main regions: an eversi-ble head, or introvert, and trunk, metamerically segmented into 11 cuticularized trunk segments. The spherical head is joined to the trunk by a short eversible neck with closing plates, called placids. The head and neck are not serially go-

mologous to trunk segments but are traditionally considered as the first and second segments; thus, there is a total of 13 body segments.

The head terminates with a protrusible mouth cone surrounded by nine oral styles. Internally, the terminal mouth is followed by 20 pentamerously arranged buccal styles, which are partly eversible when feeding. The spherical head bears 5-7 rings of as many as 89-91 posteriorly directed spines called scalids. Scalids are sensory and are also used for forward locomotion. The neck is composed of a series of closing plates, which retract over the head when it is withdrawn into the trunk. Cyclorhagids typically have 14-16 radially arranged placids. Homalorhagids have 2-4 dorsal and 2-4 ventral trapezium-like placids.

Kinorhynchs are characterized by metamerical trunk armor (exoskeleton). The trunk segments are variously subdivided longitudinally into a series of cuticular plates (dorsal tergites and ventral sternal plates). In most Cyclorhagida, segment 3 (the first trunk segment), is entire and composed of a complete ring of cuticle. In the Semnoderidae, segment 3 is divided into bivalved plates forming a clamshell-like closing apparatus, which acts with placids to close off the inverted head. In the Pycnophyidae, segment 3 is composed of one arched dorsal plate, or tergite, and three mobile ventral, or sternal, plates. Mobile ventral plates close the anterior region of the trunk when the head is introverted. Segment 4 is entire in the Echinoderidae or subdivided into one tergal and two sternal plates by midventral and lateral articulations extending posteriorly from segment 4 through segment 13 in most other kinorhynchs. Articulations, the flexible junctions of the arthrocorial cuticle allow movement between plates and segments, as well as inversion and eversion of the head.

Trunk spines are usually located middorsally (DS), laterally (LS), midterminally (MTS), or lateroterminally (LTS). In some species, there are accessory lateral (LAS) and accessory lateroterminal (LTAS) spines, middorsal processes (MP), and modified spinelike appendages. Some spines are adhesive tubes (AT). The spines are sensorial and related to locomotion.

The internal anatomy of kinorhynchs is related to the outer segmentation, nervous system, muscles, and glandular system, which are all distinctively segmented. Simultaneous contraction of segmental dorsoventral muscles increases the pressure of the body-cavity fluid in the trunk, displaces it forward, and everts the head. Scalids move forward, plow backward through the interstices around and propel the kynorhynch forward. Special head retractor muscles retract the introvert back into the trunk in synchrony with relaxation of the dorsoventral muscles.

Essentials of Human Physiology

Essentials of Human Physiology

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