Marine invertebrate larvae swim by using cilia or muscles, or a combination of these. The effectiveness of cilia as locomotory organelles diminishes with increasing body size above 1 mm. Thus, larvae propelled by cilia are small and, owing to the small Reynold's numbers that operate in this size range, their movements are governed by viscous forces rather than inertial ones. Cilia may be distributed uniformly over the surface of the larva and (or) localized on rings, bands, arms, or lobes. During development the pattern of ciliation may change; this often increases the swimming ability of the larva, particularly its manoeuverability. In many cases, redistribution of cilia coincides with the onset of feeding behavior. The locomotory currents produced by ciliary beating or the action of swimming appendages may simultaneously convey food particles to the mouth. Muscles may have enabled some larvae to exceed the size limit imposed by ciliary propulsion and also have enabled greater swimming speeds. Invertebrate larvae that use muscular locomotion possess some form of skeleton (hydrostatic, exoskeleton. or notochord) to provide the necessary resistance for muscular contraction. The density of most marine invertebrate larvae exceeds that of seawater, therefore, they must swim to stay suspended. A wide variety of parachute structures, density-reducing devices, and passive hydrodynamic mechanisms counteract the sedimenting effects of gravity. The timing of development in some larvae is such that when the tendency to sink exceeds the ability to swim, the larva is preparing for settlement and metamorphosis.

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A wide variety of rudimentary and apparently non-functional traits have persisted over extended evolutionary time. Recent evidence has shown that some of these traits may be maintained as a result of developmental constraints or neutral energetic cost, but for others their true function was not recognized. The adipose fin is small, fleshy, non-rayed and located between the dorsal and caudal fins on eight orders of basal teleosts and has traditionally been regarded as vestigial without clear function. We describe here the ultrastructure of the adipose fin and for the first time, to our knowledge, present evidence of extensive nervous tissue, as well as an unusual subdermal complex of interconnected astrocyte-like cells equipped with primary cilia. The fin contains neither adipose tissue nor fin rays. Many fusiform actinotrichia, comprising dense striated macrofibrils, support the free edge and connect with collagen cables that link the two sides. These results are consistent with a recent hypothesis that the adipose fin may act as a precaudal flow sensor, where its removal can be detrimental to swimming efficiency in turbulent water. Our findings provide insight to the broader themes of function versus constraints in evolutionary biology and may have significance for fisheries science, as the adipose fin is routinely removed from millions of salmonids each year.

Spermiogenesis of the eupyrene sperm in the snail, Fusitriton oregonensis, was studied with light and electron microscopes. Endoplasmic reticulum, which encircles the nucleus in each spermatid, appears to connect with the Golgi body and to interconnect between adjacent spermatids via cytoplasmic bridges. It is suggested that as the Golgi body migrates around the nucleus the endoplasmic reticulum may circulate with it. The alignment of the proacrosome with the nucleus is effected by a 180° rotation of the Golgi body, after which it separates and migrates posteriorly with the residual cytoplasm. Each sperm possesses a well‐developed intracellular digestive system as indicated by multivesicular bodies, residual bodies, and myeloid figures. Autophagy begins in the residual cytoplasm before it is released from the middle piece. Microtubules are found outside the nucleus and mitochondria during the final stages of spermiogenesis, when elongation is almost complete. These microtubules appear to be involved in the final shaping and twisting process, in which torsion is locked in the nucleus and the mitochondria spiral around the axoneme. The annulus attaches the distal centriole to the plasma membrane in the early spermatid and as flagellar production begins they move towards the implantation fossa at the base of the nucleus. There are two centrioles in the early spermatid, the distal centriole and procentriole. The small procentriole fuses with the distal centriole in the intranuclear canal to form the centriolar cap of the basal body. This cap is pushed through the end of the nuclear tube and is separated from the subacrosomal space by only the nuclear membranes.

Nurse cells develop from diploid cells in the testis. Each cell undergoes a reduction division which leaves the nucleus with half the volume of a normal diploid cell. They send out pseudopodia which form desmosome-like junctions with developing spermatids. The nurse cells detach from the testicular wall, their nuclei degenerate and secretion droplets form in the cytoplasm. The pseudopodia are drawn in as the cytoplasmic secretions swell and the nurse cell becomes spherical. The eupyrene sperm become grouped unilaterally and at this stage are attached to the nurse cell by only the tips of their acrosomes. At maturity the nurse cells with their clumps of attached eupyrene sperm (spermatozeugmata) are released from the testis via ducts into the seminal vesicles,where they are stored prior to copulation. Nurse cells serve similar functions to those of apyrene sperm which are common among the Molluscs. We believe that the nurse cell and apyrene sperm are homologous.