William Abbott Herdman

“ The ova (always few in number, usually only one) appear at a very early period in the developing chain Salpa, while it is still a part of the gemmiparous stolon in the body of the solitary Salpa. This gave rise to the view put forward by Brooks (25) , that the ovary really belongs to the solitary Salpa, which is therefore a female producing a series of males by asexual gemmation, and depositing in each of these an ovum, which will afterwards, when fertilized, develop in the body of the male into a solitary or female Salpa. ”

“ The chief genera are: Molgula (Forbes) , with distinct folds in the branchial sac, and Eugyra (Ald. and Hanc.) , with no distinct folds, but merely broad internal longitudinal bars in the branchial sac. In some of the Molgulidae (genus Anurella, Lacaze-Duthiers, 20) the embryo (fig. 14, M) does not become converted into a tailed larva, the development being direct, without metamorphosis. ”

“ Throughout the rest of the mantle the bands of muscle fibres form a rude irregular network. They are numerous on the right side of the body, and almost totally absent on the left. The muscles are all formed of, very long usiform non-striped fibres. The connective tissue of the mantle is chiefly a clear gelatinous matrix, containing cells of various shapes; it is frequently pigmented, giving brilliant red or yellow colours to the body, and is penetrated by numerous lacunae, in which the blood flows. ”

“ The later Proto-Ascidiacea were probably colonial forms, and gemmation was retained by the Clavelinidae and by the typical compound Ascidians (Distomidae, &c.) derived from them. The power of forming colonies by budding was lost, however, by the primitive simple Ascidians, and must, therefore, have been regained independently by the ancestral forms of the Botryllidae and the Polystyelidae. If this is a correct interpretation of the of evolution of the Tunicata, we arrive at the following important conclusions. (1) The Tunicata, as a whole, form a degenerate branch of the Proto-Chordata ”

“ The cells of the latter hemisphere divide more rapidly, and form the future aboral surface. When the dorsal hemisphere has twenty-two cells the ventral has fifty-four. The gastrulation is a combination of epiboly and invagination. The ventral ectoderm grows over, so as to envelop the dorsal hemisphere, while the latter sinks down and becomes saucer-shaped. In the centre of the dorsal surface ten cells form the future endoderm. ”

“ The buds give rise eventually to the sexual generation, which is polymorphic, having three distinct forms, in two of which the reproductive organs remain undeveloped. The buds while still very young migrate from their place of origin on the stolon, divide by fission, and become attached to the dorsal outgrowth of the body of the nurse, where they develop. ”

“ It is now known to be a degenerate branch of the Chordata, and to be more nearly related to the Vertebrata than to any group of the Invertebrata. The Tunicata are found in all seas, from the littoral zone down to abyssal depths. They occur either fixed or free, solitary, aggregated or in colonies. The fixed forms are the “simple” and “compound” Ascidians. The colonies are produced by budding and the members are conveniently known as Ascidiozooids. Some Tunicata undergo alternation of generations, and most of them show a retrograde metamorphosis in their life-history. ”

“ Some of the buds Reproduction by Gemmation and the Formation of Colonies. have an abundant store of reserve materials in their outer layer of cells, while others are without this supply. The former are placed deeply in the stalk, develop slowly, and probably serve to regenerate the colony when the head portion has been removed or has died down. In these cases where the ectoderm has taken on the function of storing the reserve material, it is found that all the organs of the bud are formed from the cells of the endodermic vesicle. ”

“ Family VII., Polystyelidae.—Ascidiozooids not grouped in systems. Branchial and atrial apertures four-lobed. Branchial sac may be folded; internal longitudinal bars present. The chief genera are: Thylacium (Carus) , with ascidiozooids projecting above general surface of colony; Goodsiria (Cunningham) , with ascidiozooids completely imbedded in investing mass; and Chorizocormus (Herdman) , with ascidiozooids united in little groups which are connected by stolons. Several of the species show transitions between the other Polystyelidae and the Styelinae amongst simple Ascidians. ”

“ Various functions have been assigned to these ampullae in the past, and Bancroft has shown that in addition to acting as storage reservoirs for blood, organs for the secretion of test matrix, and accessory organs of respiration, they are also organs for blood propulsion. ”

“ The outer grey part of the body, which is attached at or near its The Test. posterior end and penetrated by the two apertures, is the “test.” This is a firm gelatinous cuticular secretion upon the outer surface of the ectoderm, which is a layer of flat cells. Although at first produced as a cuticle, the test soon becomes organized by the migration into it of cells derived from the mesoderm. A. Kowalevsky has shown that cells of the mesenchyme of the larva make their way through the ectoderm to the exterior during the metamorphosis, and become the first cells of the young test. ”