Amniote phylogeny and the importance of fossils

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Silicon and siliceous structures in biological systems

Attachment devices of representatives of most higher taxa of hexapods were examined. Short descriptions of tibial, tarsal and pretarsal adhesive structures for each order are presented. In their evolution, hexapods have developed two distinctly different mechanisms to attach themselves to a variety of substrates: hairy surfaces and smooth flexible pads. The flexible properties of pad material guarantees a maximal contact with surfaces, regardless of the microsculpture. These highly specialized structures are not restricted to one particular area of the leg. They may be located on different parts, such as claws, derivatives of the pretarsus, tarsal apex, tarsomeres, or tibia. The 10 characters of the two alternative designs of attachment devices – smooth and hairy – were coded and analysed together with a data matrix containing 105 additional morphological characters of different stages and body parts. The analysis demonstrates, that similar structures (arolium, euplantulae, hairy tarsomeres) have evolved independently in several lineages. Nevertheless, some of them support monophyletic groups (e.g. Embioptera + Dermaptera; Dictyoptera + Phasmatodea + Grylloblattodea + Orthoptera; Dictyoptera + Phasmatodea; Hymenoptera + Mecopterida; Neuropterida + Strepsiptera + Coleoptera). Other structures such as claw pads (Ephemeroptera), balloon-shaped eversible pads (Thysanoptera), or fossulae spongiosae (Reduviidae) are unique for larger or smaller monophyletic units. It is plausible to assume that the evolution of flight and the correlated necessity to cling to vegetation or other substrates was a major trigger for the evolution of adhesive structures. Groups with a potential to evolve a great variety of designs of adhesive pads are Hemiptera and Diptera. Even though characters of the adhesive pads are strongly subject to selection, they can provide phylogenetic information. The results of the cladistic analyses are largely congruent with current hypotheses of hexapod phylogeny. A sistergroup relationship between Diplura and Insecta and between Zygentoma (excl. Tricholepidion) and Pterygota is confirmed. Plecoptera are probably the sistergroup of the remaining Neoptera. Dermaptera are the sistergroup of Embioptera and Dictyoptera the sistergroup of Phasmatodea. Paurometabola excl. Dermaptera + Embioptera are monophyletic. A sistergroup relationship between Zoraptera and a clade comprising Paraneoptera + Endopterygota is only supported by weak evidence. Coleoptera + Strepsiptera are the sistergroup of Neuropterida and Hymenoptera the sistergroup of Mecopterida.

Ultrastructure of attachment specializations of hexapods (Arthropoda): evolutionary patterns inferred from a revised ordinal phylogeny

The Unicellular Ancestry of Animal Development

Summary
1. The exchange of oxygen and carbon dioxide between skin and environment is commonplace in the vertebrates. In many lower vertebrates, the skin is the major or even sole avenue for respiration.

2. As implied by the physical laws governing diffusion of gases, the skin diffusion coefficient, surface area, gas diffusion distance and transcutaneous gas partial pressures may independently or jointly affect cutaneous respiration. In vertebrates, each of these variables has undergone modification that may be related to dependence upon cutaneous gas exchange.

3. Both theoretical models and experimental data suggest that cutaneous gas exchange is limited by the rate of diffusion. However, changes in convection of the respiratory medium and of blood may partially compensate for diffusion limitation, and potentially function in the regulation of cutaneous gas exchange.

4. Typically, the skin is one of several gas exchangers, although many salamanders and some species in other vertebrate groups breathe solely through the skin. The cutaneous contribution to overall gas exchange is often most important in small animals, at cool temperatures, at low levels of activity and in normoxic and normocapnic conditions. Branchial and pulmonary respiration increasingly predominate in other circumstances.

5. Often, the skin figures more prominently in CO2, excretion than in O2, uptake.

6. Cutaneous gas exchange emerges in vertebrates as a process perhaps less effective and more constrained than branchial or pulmonary exchange but also less energetically costly. Its utility is indicated by its wide and successful exploitation in vertebrates occupying a diverse array of habitats.

Cutaneous Gas Exchange in Vertebrates: Design, Patterns, Control and Implications

Choanocyte-like cells with a collar of regularly arranged cylindrical microvilli around the base of the flagellum were observed in the ciliary bands of the Brachiolaria larva of Asterias rubens. The ambulacral ampullae and coelomic epithelia of adult Asterias and coelomic epithelia of Mesothuria contain similar cells with radial lamellae instead of cylindrical microvilli. Other similar but more modified types of cells, in which the inner edges of the radiating lamellae could be recognized as longitudinal ridges in the wall of a cylindrical flagellar pit, were found in ambulacral ampullae of Porania and in coelomic epithelia of Stichopus. Distinct indications of phagocytosis were seen in most of these cells. The present report together with previously published data lend support to the idea, that the choanocyte is a fundamental cell type in metazoans, probably derived phylogenetically from some flagellate ancestor.

On the Occurrence and Structure of Choanocyte-like Cells in Some Echinoderms

The spermatozoa of Petrobius and Lepisma share a few general insect features (filamentous shape, two mitochondria, compact acrosome vesicle, bilateral symmetry) but differ fundamentally with regard to specializations. In Petrobius, a long coiled acrosome, a coiled nucleus, and a “body” with axonema, two mitochondria, and a pair of lateral bodies follow each other in normal sequence. In Lepisma the acrosome is a small vestige in the spoon-shaped anterior end, the centriole is dislocated anteriorly, and nucleus, two mitochondria and axonema run like parallel filaments through most of the spermatozoon. The centriole adjunct develops into a postnuclear body in Lepisma but forms a pair of complicated “lateral bodies” in Petrobius. It is concluded that ancestral forms must have had fairly primitive spermatozoa and that specialization has proceeded independently within each evolutionary line.

The Spermatozoa of the Thysanuran Insects Petrobius brevistylis Carp. and Lepisma saccharina L.

The structure of the intestine and its adnexa in the Australian lungfish Neoceratodus forsteri (Krefft), is described from four adult specimens and compared with that of other lungfish. In all lungfish genera the intestine is thick and straight and contains a complicated spiral valve; a pyloric fold separates the foregut from the midgut. In Neoceratodus the coiling of the intestinal lumen begins in the prepyloric or gastric region, unlike in other vertebrates with a spiral valve, where it begins behind the pylorus. The spiral valve of Neoceratodus begins as a deep groove on the right side of the foregut, just behind the glottis. Such a prepyloric groove is present but poorly developed also in the lungfishes Protopterus and Lepidosiren and contains a prepyloric spleen in all genera. A separate postpyloric spleen is situated in the free margin of the spiral valve, i.e. in the axis of the intestine. The spleen has a heavily pigmented cortex containing large amounts of iron. The pancreas, buried in the spiral valve in front of the pylorus in Neoceratodus, contains numerous islets of Langerhans, similar to those of tetrapods. This is unexpected because the islets ofProtopterus, described by Gabe in 1969, are more like the large, encapsulated “Brockmann bodies” of teleosts.

Structure of Intestine, Pancreas, and Spleen of the Australian Lungfish, Neoceratodus forsteri (Krefft)

A sphincter is described in the a. pulmonalis in the frog Rana temporaria. The sphincter begins abruptly where the pulmonary artery branches off from the a. pulmo-cutanea. It is constricted by vagal stimulation and acetylcholine. It also contracts actively when the lungs are artificially collapsed and opens when the lungs are inflated. It is suggested that previously reported fibers in the vagus nerve, which come from stretch receptors in the lungs, act as the afferent reflex arch. When the pulmonary artery constricts, the cutaneous artery and vein are distended by increased pressure which contributes to increased cutaneous flow. The sphincter therefore seems to play an important role in the adjustment of circulation to exclusive cutaneous respiration as for instance in submerged, wintering frogs. It is suggested that the decrease of pulmonary flow in such situations causes increased mixing in the heart and, perhaps, complete break-down of the double circulation. In frogs with exclusive skin respiration this would seem to be a functional necessity. Sphincters of similar location in turtles seem to be homologous with the one in Rana.

A Sphincter in the Pulmonary Artery of the Frog Rana temporaria and Its Influence on Blood Flow in Skin and Lungs

The paper deals with the erythrocytes of the teleostean fish, Maurolicus mulleri (Gmelin). Three remarkable features in their morphology are pointed out:

1.

They have no nuclei. Whether nucleated erythrocytes exist in the blood could not be stated with certainty. Nucleated cells make up less than 1 % of the blood corpuscles and do not show the staining properties of mature erythrocytes.




2.

They are exceptionally small, 4–6 × 2–3 μ. So small erythrocytes are found only in a few mammals.




3.

They do not seem to be flattened. It is suggested that this is correlated with their small size.

Karl Georg Wingstrand

Papers

On the Occurrence and Structure of Choanocyte-like Cells in Some Echinoderms

The Spermatozoa of the Thysanuran Insects Petrobius brevistylis Carp. and Lepisma saccharina L.

Structure of Intestine, Pancreas, and Spleen of the Australian Lungfish, Neoceratodus forsteri (Krefft)

A Sphincter in the Pulmonary Artery of the Frog Rana temporaria and Its Influence on Blood Flow in Skin and Lungs

Non-nucleated erythrocytes in a teleostean fish Maurolicus mülleri (Gmelin).