Tadpole larvae are found in three orders and 10 families of ascidians (phylum Chordata, subphylum Urochordata or Tunicata) and range in length from about 0.6 mm to 11.0 mm. They all have a trunk, a tail, and cuticular fins. The largest larvae are produced by colonial ascidians (with asexual reproduction) and the smaller ones are produced by solitary ascidians (without asexual reproduction). The body plan is similar in all species, but there are striking differences in the histology of the organs and organ rudiments (Figure 1). The principal parts can be categorized as transitory larval organs (TLO), prospective juvenile organs (PJO), and larval-juvenile organs (LJO) (see box). Organs in the first group (TLO) function in larval locomotion, sensory input, and settlement. The TLO are fully differentiated and functional in the larva, but they are destroyed or lost during metamorphosis. Organs in the second group (PJO) may be fully differentiated or rudimentary in different species. They may become functional within minutes or hours after settlement, as in most colonial ascidians, or following several days of further histogenesis, as in most solitary ascidians. The last group (LJO) includes organs and tissues that function in both larval and post-larval phases of the life cycle. The ultrastructure of the TLO and the LJO has been described in a few species. Little work has been done on the PJO. The events of metamorphosis that transform the nonfeeding, mobile larva into a filter-feeding, fixed juvenile are commonly as follows. * Secretion of adhesives by the papillae or the epidermis of the trunk * Eversion and retraction of the adhesive papillae