A comparative analysis of the larval and presumptive juvenile neuromuscular systems among actinotroch larvae was performed using confocal laser microscopy with probes for F-actin and serotonin. Currently, there are two main categories of larval nervous systems based on the origin of the nerve fibers that innervate the larval tentacles. Characteristics of the serotonergic cells of the larval apical ganglion and juvenile nervous system have remained relatively conserved, but the structure of the secondary (hood) sense organ and the juvenile tentacles has diversified among species. Differences in larval musculature are mainly associated with differences in hood morphology. The presumptive, juvenile neuromuscular system is either integrated or separated from that of the larva based on the origin of the juvenile tentacles. Among species, the juvenile tentacles are made by remodeling the larval tentacles, developed from a basal tentacular thickening, or developed as a completely separate set in the larva. Differentiation of the neuromuscular structures of the juvenile tentacles is more diverse than their outward morphological characteristics would suggest. Importance of these larval characters is discussed in terms of current problems that exist within phoronid systematics. Evolutionary implications of these morphological characters are discussed among the phoronids, brachiopods, and related bilaterians. Overall, the integration or separation of larval and juvenile neuromuscular characters may yield insights into the evolution of lophotrochozoan body plans.
Embryos of the marine cheiloctenostome bryozoan Bugula neritina undergo a marked increase in volume (about 500-fold) during embryogenesis while being retained in a brood chamber. Previous morphological studies indicate that shortly after transfer of the zygote to the brood chamber, the epithelium of the maternally-derived portion of the brood chamber, the ooecial vesicle, differentiates in regions adjacent to the embryonary space from a squamous to a columnar form suggesting that the parent is involved as a source of extraembryonic nutrients required for the extensive growth of the embryo.Results of the present ultrastructural study indicate that hypertrophy of the epithelial cells occurs only in that region of the ooecial vesicle which opposes the embryo, that differentiation (and subsequent regression) of the lining are predictable events correlated with the onset (and termination) of embryonic growth, and that hypertrophied cells are well equipped for the synthesis and transport of macromolecular materials across the vesicle wall to the developing embryo. Further, that portion of the embryo's ectoderm (the presumptive metasoma1 sac) in contact with this hypertrophied epithelium is morphologically specialized for the uptake of nutrients. Finally, shortly before release of the larva, this intimate association of the metasomal sac tissue and the hypertrophied ooecial vesicle lining epithelium is terminated by invagination of the sac and atrophy of the lining.In the sexual reproduction of metazoans, two divergent strageties have evolved: (1) the production of vast numbers of freely spawned small eggs that will develop to larvae which feed for a variable period of time before metamorphosis to the adult form; and (2) the formation of a small number of ova provided either with a large yolk reserve during oogenesis or with nutrients from the parent during embryogenesis.A broad spectrum of types of extraembryonic nutrition systems have evolved independently within the animal kingdom, but very few of these have been analyzed. only in higher vertebrates is there an extensive literature on the morphological and physiological specializations of the parental and embryonic components involved in placental transfer of nutrients (see Asdell, '64 for review). In these cases J. MORPH., 1 4 7 . 355-378.
The structure, attachment and subsequent metamorphosis of larvae of the marine bryozoan Bugula neritina were studied by light and electron microscopy.Two points of larval anatomy are of special significance to proper interpretation of the metamorphosis :1. Two cytologically similar blastemal tissues, each laden with free ribosomes, occur as parts of the apical organ complex. The upper blastema directly contacts the larval surface, forming the non-ciliated rows of the apical organ. The lower blastema is internal and is oral to and contiguous with the upper blastema. 2. The epidermal tissues of the larva are joined in the following sequence, beginning at the aboral pole: a. apical organ complex; b. apical-connecting cell; c. infolded pallial sinus epithelium; d. vesicular-connecting cell; e. aboral vesicular epithelium; f. corona; g. oral vesicular epithelium; and i., j., and k. internal sac neck, wall and roof regions.The initial stages of metamorphosis involve a complex sequence of morpho-1. eversion of the internal sac, permanently attaching the larva to the substrate; 2. inrolling of the aboral vesicular epithelium, corona, oral vesicular and ciliated epithelia, and neck region of the internal sac into the larval interior; concomitantly the pallial sinus epithelium evaginates; 3. loss of connection between the invaginated tissues and the surface; 4. fusion of the pallial sinus epithelium with the wall region of the internal sac, maintaining the integrity of the body surface; 5. retraction of the apical organ complex and invagination of the pallial sinus epithelium with the simultaneous elevation of the internal sac wall region to the aboral pole. At the conclusion of these events the preancestrular surface is covered by the wall and roof regions of the internal sac. Cells of the wall region form the epidermis of the body wall except for the attachment disc and secrete a cuticular exoskeleton that is secondarily calcified; the attachment disc is formed by the roof region of the internal sac.Internally, the ectodermal upper blastema differentiates into the lophophore and digestive tract of the ancestrular polypide, while the lower blastema forms the lining of the lophophoral coelom and the splanchnic (but not the somatic) lining of the visceral coelom. The visceral somatic peritoneum is formed from cells that may originate from the mesodermally derived pigmented cells of the larva to which they are similar in pigmentation and cytology. Such a composite derivation of a coelomic lining has not been described previously. genetic movements, including:The cheilo-ctenostome bryozoan Bugula neritina (Linnk) is an important contributor to marine fouling in temperate and tropical waters around the world (Ryland, '65). Due to its significance in fouling, the reproductive biology of this species has received considerable attention. The seasonality and abundance of this species has been considered by Edmonson and Ingram J. MORPH., 134: 351-382.
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