Summary (1) Based on the classical studies of Goodrich, protonephridia are believed to be phylogenetic antecedents of metanephridia. It is argued here that the primary factor determining the type of nephridium expressed is body size rather than phylogenetic status. (2) The proposed model defines a nephridium functionally and predicts two general configurations for filtration nephridia in animals. (3) Application of the model to metanephridial and protonephridial systems indicates differences in the sites of ultrafiltration and mechanisms of pressure generation. (4) Metanephridial systems function by muscle‐mediated filtration of vascular fluid into a coelomic space before modification by an excretory duct. (5) Protonephridial systems function by cilia‐mediated filtration of extracellular fluid into the lumen of a protonephridial terminal cell before modification in an adjoining duct. (6) The model predicts a correlation between animals with blood vessels and metanephridia, and animals without blood vessels and protonephridia. The correlation is shown to be nearly perfect. (7) Exceptions to the model are discussed. (8) Original experimental evidence is given for the permeability of the protonephridial terminal cell to iron dextran and its reabsorption by the protonephridial duct in the polychaete, Glycera dibranchiata. (9) Experimental data for proto‐ and metanephridial systems are summarized and shown to support the proposed model. (10) The ultrastructure of the exceptional amphioxus ‘protonephridium’ is reviewed and original data are presented. Its organization is structurally and perhaps functionally intermediate between proto‐ and metanephridial systems. (11) An original ultrastructural comparison is made of monociliated nitration cells in a size range of larval invertebrates from five phyla. Filtration cells that are structurally intermediate between protonephridial solenocytes and metanephridial podocytes are noted in larvae intermediate in body size between the two extremes. The comparative data suggest that (i) podocytes and solenocytes are homologous cells and (ii) that body size is correlated with which of the two designs is expressed. (12) The fates of larval podocytes are followed through metamorphosis in three species. The results confirm the equivalence of podocytes and solenocytes as suggested by the comparative analysis. They further indicate that which morph is expressed is a function of body design factors discussed in the model. (13) Protonephridia are believed to be primitive to metanephridia because they occur in presumably primitive animals and in ontogenetic stages of many animals with metanephridia as adults. It is suggested here that the distribution of protonephridia is related to small body size and the lack of blood vessels, regardless of phylogenetic status. The occurrence of protonephridia in the larvae of species with metanephridia as adults is explained similarly as a function of the small larval size and lack of blood vessels.

A comparative structural and ultrastructural investigation is made of genital organs of selected, genera of Thaumastodermatidae and their constructions are defined. Two new subfamilies are designated within the family based on reproductive features and other characteristics. The occurrence of bounding epithelia about the gonads, ova and central body region is noted. Paramyosin type muscles are noted for Platydasys. General and specific features of spermatogenesis, oogenesis and fertilization are defined and discussed. The evolution of accessory reproductive organs in Macrodasyida is discussed.

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Four chordate characters -dorsal hollow nerve cord, notochord, gill slits, and endostyle -are compared morphologically, molecularly, and functionally with similar structures in hemichordates to assess their putative homologies. The dorsal hollow nerve cord and enteropneust neurocord are probably homoplasies. The neurocord (= collar cord) may be an autapomorphy of Enteropneusta that innervates a unique pair of muscles, the perihemal coelomic muscles. Despite the apparent lack of organ-level homology, chordates and enteropneusts share a common pattern of neurulation that preserves a "contact innervation" between neuro-and myo-epithelia, which may be the primitive deuterostome pattern of neuromuscular innervation. The chordate notochord and hemichordate stomochord are probably homoplasies. Other potential notochord antecedents in hemichordates are examined, but no clear homolog is identified. The comparative morphology of notochords suggests that the "stack-of-coins" developmental stage, retained into adulthood only by cephalochordates, is the plesiomorphic notochord form. Hemichordate and chordate gill slits are probably homologs, but only at the level of simple ciliated circular or oval pores, lacking a skeleton, as occur in adults of Cephalodiscus spp., developmentally in some enteropneusts, and in many urochordates. Functional morphology, I 125 -binding experiments, and genetic data suggest that endostylar function may reside in the entire pharyngeal lining of Enteropneusta and is not restricted to a specialized midline structure as in chordates. A cladistic analysis of Deuterostomia, based partly on homologs discussed in this paper, indicates a sister-taxon relationship between Urochordata and Vertebrata, with Cephalochordata as the plesiomorphic clade.Résumé : Nous comparons quatre caractères des chordés -la corde nerveuse dorsale creuse, la notochorde, les fentes branchiales, l'endostyle -des points de vue morphologique, moléculaire et fonctionnel aux structures similaires chez les hémichordés afin d'évaluer les homologies putatives. La corde nerveuse dorsale creuse et la neurocorde des entéropneustes sont probablement des homoplasies. La neurocorde (= corde du collier) peut être une autapomorphie des entéropneustes qui innerve une paire spéciale de muscles, les muscles du coelome périhémal. Malgré l'absence apparente d'homologies entre les organes, les chordés et les entéropneustes possèdent en commun un même système de neurulation qui conserve une « innervation par contact » entre les épithéliums nerveux et musculaires, ce qui peut être un arrangement primitif de l'innervation neuromusculaire chez les deutérostomiens. La notochorde des chordés et la stomochorde des hémichordés sont probablement des homoplasies. Nous avons examiné d'autres antécédents potentiels de la notochorde chez les hémichordés, sans toutefois trouver d'homologie claire. La morphologie comparée des notochordes laisse croire que le stade de développement « en empilement de pièces de monnaie », retenu à l'état adulte seulement chez les cé...