Order II.—Thaliacea.

The Thaliacea are free-swimming Tunicates, which exhibit alternation of generations in their life history. There are three families, Salpidæ, Octacnemidæ, and Doliolidæ.

Salpidæ.—The Salpas are transparent barrel-shaped organisms which occur in abundance at the ocean surface. They are so transparent that they are rarely seen, except in calm weather from the side of small boats; yet they frequently swarm in countless multitudes. From five to ten bands of muscles partially or entirely surround the body, like hoops. The branchial and atrial openings are at or near the opposite ends of the body. The branchial sac has almost disappeared, the dorsal lamina and ventral gutter (or endostyle) alone remaining, the interval between the two on each side representing an enormous stigma; the dorsal lamina, or “gill” is the transversely striated band passing obliquely across the body and forming the only barrier between the branchial and atrial cavities. Water enters at the mouth, and, by the contraction of the muscle-hoops, is driven out through the atrial aperture at the opposite end, which is then closed by a sphincter muscle. The elastic walls of the body expand, and water again enters through the mouth, the valve-like lips of which prevent its being driven out that way. The Salpa swims along in jerks, and along with each gulp of water takes in Radiolaria, Foraminifera, etc., which are retained by the mucus of the endostyle and carried to the gullet. The Salpa, in fact, lives, as Professor Brooks observes, in a “living broth,” so abundant is the food supply.

The intestines usually form an oval mass termed the “nucleus,” which is a conspicuous object at the posterior end.

Fig. 21.
Posterior part of solitary form of Salpa democratica-mucronata, showing a chain of embryos nearly ready to be set free.
gem, young chain of Salpæ; st, stolon; t, test; visc, visceral mass.

The solitary Salpa above described is asexual. In the ventral region of its body it forms a stolon which becomes segmented into a series of buds (Fig. 21). As the stolon grows the end series of buds breaks off in the form of a chain and swims away, other chains being detached in succession. A chain is formed of individuals arranged in two rows, the individuals in each row being alternate (not opposite).

Each individual of a chain differs from the solitary individual in shape, arrangement of muscle bands, etc., but especially in having reproductive organs. The chain Salpid is hermaphrodite; the embryo develops into a solitary asexual Salpa which produces the chains by budding. The wonderful life history of Salpa was discovered by the poet Chamisso during a voyage round the world in 1819. He observes: “A Salpa mother is not like its daughter or its own mother, but resembles its sister, its granddaughter, and its grandmother.” Here we have an example of “alternation of generations,” a sexual generation (chain form) giving rise to an asexual generation (solitary form), which latter produces the sexual generation.[^[23]]

Most of the species of Salpa have double names owing to the chain and solitary forms having been regarded as distinct species before they were known to be phases in the life history of one and the same species. Salpa runcinata-fusiformis, solitary form (Fig. 22 B), is barrel-shaped, truncated at each end, with terminal orifices, and with nine muscle-bands on the dorsal surface, some of which converge towards each other. An individual of a chain (Fig. 22 A) is fusiform, with six muscle-bands, and with the orifices not terminal, but at each end of the dorsal surface.

The solitary form of S. africana-maxima is barrel-shaped, with truncated ends and terminal orifices, and with nine broad parallel muscle-bands. The chain form is conical at one end, with six bands, and with orifices on the dorsal surface. The exhibited specimen of the chain form, which is in an early stage of growth, contains 202 individuals. The solitary and chain individuals of Salpa costata-tilesii attain a length of six to eight inches. The solitary form has eighteen muscle-bands and two large spines at the posterior end. The individual of the chain has five muscle-bands. A chain of three individuals is exhibited.

Salpa pinnata produces a circular chain; the exhibited specimen of the solitary form shows a small chain about to be detached; a circular chain of six individuals is also exhibited. Species of Salpa abound in all seas, but specimens from Naples have alone been exhibited on account of their good preservation.

Family Octacnemidæ includes O. bythius, a deep-sea Salpid, in which the body forms a flattened disk produced into eight radiating lobes.

Fig. 22.
Salpa runcinata-fusiformis. A. Chain form. B. Solitary form. 1–9, muscle bands; em, embryo; m, mantle; visc, visceral mass or nucleus.
(Herdman: Tunicata, Encyclopædia Britannica.)

Fig. 23.
Doliolum denticulatum, sexual generation, from the left side. m¹-m⁸ muscle bands; at, atrial; br, branchial apertures; br s, branchial sac; sg, stigmata; st, stomach; ng, nerve ganglion; so, sense organs.
(After Herdman, Encyclopædia Britannica.)

Family Doliolidæ. The body is cask-shaped and surrounded by circular hoops. The branchial and atrial orifices are at the opposite ends. The branchial sac is pierced by two oblique bands of stigmata (Fig. 23 sg). The life history is very complicated. The egg develops into a tailed larva, which develops into a “nurse”; the latter is asexual, and produces three kinds of buds on a stolon, viz. (1) nutritive buds which provide the “nurse” with food, (2) foster forms which are set free as cask-shaped bodies with eight broad muscle-bands, and (3) sexual forms which are attached for a time to the foster forms, but which later become free and give rise to the egg.