Fig. 32.—Diagram of Fritillaria seen from the right side to show the elongated body, the hood, and the relative positions of anus, atrial opening, and gonads. (Compare with Oikopleura, Fig. 30.) a, Anus; at, opening of atrial tube; br.s, branchial sac; end, endostyle; ht, heart; m, mouth; n.ch, notochord; n.g, nerve-ganglion; oes, oesophagus; ov, ovary; sg, stigma; sp, testis; st, stomach.
Order II. Ascidiacea (Ascidians).
Fixed or free-swimming Simple or Compound Ascidians, which in the adult are never provided with a locomotory appendage or tail, and have no trace of a notochord. The free-swimming forms are colonies, the Simple Ascidians being always sedentary and usually fixed. The test is permanent and well developed, and becomes organised by the immigration of cells from the body; as a rule it increases in size with the age of the individual. The branchial sac is large and well developed. Its walls are perforated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the single atrial aperture. Many of the Ascidiacea, both fixed and free, reproduce by gemmation to form colonies, and in most of them the sexually produced embryo develops into a tailed larva.
The Ascidiacea includes three groups, the Simple Ascidians, the Compound Ascidians, and the free-swimming colonial Pyrosoma, which in some respects connects this Order with the Thaliacea.
Sub-Order 1. Ascidiae Simplices.
Fixed Ascidians, which are solitary, and very rarely reproduce by gemmation; if, as in a few cases, small colonies are formed, the members are not buried in a common investing mass, but each has a distinct test of its own. No strict line of demarcation can be drawn between the Simple and Compound Ascidians; and one of the families of the former group, the Clavelinidae (the "Social" Ascidians of Milne-Edwards), forms a transition from the typical Simple forms which never reproduce by gemmation, to the Compound forms which always do. Over 500 species of Ascidiae Simplices are now known, but there are probably very many more still undescribed. The sub-order may be divided into the following families:—
Fam. 1. Clavelinidae.—Simple Ascidians which reproduce by gemmation to form small colonies (Fig. 33), in which each member, or ascidiozooid, has a distinct test, but all are connected by a common blood-system, and by a prolongation of the "epicardiac tubes" (see p. [83]) from the branchial sac. Buds are formed on the stolons (Fig. 33), which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm, and endoderm (the epicardium) of the Ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. The Clavelinidae are the simplest of the Ascidiae Simplices. They are the forms that come nearest to the Compound Ascidians, and are closely related to the Distomatidae. They are probably the nearest representatives now existing of the ancestral forms from which both Simple and Compound Ascidians are descended.
Fig. 33.—Colony of Clavelina lepadiformis (nat. size).
This family contains amongst others the following three genera:—Ecteinascidia, Herdman, with internal longitudinal bars in the branchial sac; Clavelina, Savigny, with a long body and intestine extending behind the branchial sac (Fig. 33); and Perophora, Wiegmann, with a short compact body and intestine alongside the branchial sac. Clavelina lepadiformis and Perophora listeri are common British species found at a few fathoms depth off various parts of our coast. Both occur round the south end of the Isle of Man. In autumn Clavelina accumulates reserve-material in the ectoderm cells of parts of the stolon, which remain when the rest of the colony dies away, and then form new buds in spring.