Only genus and species, Appendicularia sicula, Fol,—body <0.5mm, house < 2.6mm. in greatest diameter.
Sub-fam. ii. Oikopleurinae.—Anus mid-ventral, between or in front of spiracles. Oesophagus dorsal. House much expanded, usually with lateral filtering windows; caudal chamber horizontal, tail pointing forwards.
Oikopleura.—Stomach bilobed, clamped between dorsal oeso phagus and ventral rectum. Mouth with projecting under-lip. The species in general fall into two sections, which differ in details of the oikothelium—(i.) fusiformis, longicauda, etc. without oral glands, and (ii.) labradoriensis, albicans, dioica, etc. with oral glands. 0 longicauda (=spissa, Fol.) is the only species with a postero-dorsal "veil," and a median posterior inhalant aper ture to the house, without filtering windows.
In various oceanic types related to the second group, but with gelatinized blastocoele, the intestinal loop opens out, either (i.) vertically (e.g., Folia, Stegosoma, etc.) or (ii.) horizontally (viz., Bathochordaeus and Althoffia). In several respects Batho chordaeus approaches Appendicularia. The houses of all are un known. Only Folia and Stegosoma possess oral glands.
Fam. II. Fritillaridae.—Stomach and intestine reduced (oligo cytic) and vesicular; anus behind spiracles, on right side. Endo style without flagella, nearly closed. Genital region produced, blastocoele gelatinised. Oikothelium short and narrow, covered by a dorsal hood arising far forward. No house, but an elastic filtering capsule, projected over the mouth from the hood-cavity.
Fritillaria.—Hood not produced in front of snout. The num erous species mostly fall into two groups: (i.) with tail forked, mouth complicated, usually a "pharyngeal packet" behind endo style, e.g., borealis, pellucida (= furcata, Fol.), and (ii.) with tail pointed, mouth structure very simple, pharyngeal packet com pletely wanting or when present very greatly reduced to a small number of peculiar cells, e.g., haplostoma.
Tectillaria.—Hood greatly produced and pointed, arising in front of spiracles, and bearing the far greater part of the oikothe lium on its lower fertilis, Lohm.
Sub-order II. POLYSTYLOPHORA (Garstang, 1895). Endostyle
absent. Spiracles slit-like. Peripharyngeal and ventral ciliated bands replaced by 4 longitudinal rows of ciliated processes, forming an endopharyngeal sieve. House capacious, bi-radially symmetrical round a single aperture.
Fam. Kowalevskidae.—Oikothelium mainly dorsal, radially arranged around a central boss, under a delicate hood arising far back (Lohmann, 1896). Oesophagus ventral, gut bi-vesicular, stomach left, rectum and anus right.
Only genus and species, Kowalevskia tennis, Fol. (1872).
If the preceding classification accurately resumes the facts of structure and the affinities of the numerous living forms of Tunicata, it presents some remarkable features : the three orders stand at three different morphological levels, but in descending, instead of ascending, order. The Ascidiacea with their trans verse rows of stigmata are followed by the Thaliacea, with un divided protostigmata, and these by the Copelata with a single pair of simple perforations. The situation is much the same with regard to atrial cavities and endostyle.
The following considerations appear to justify the order in question as representing a true phyletic sequence. The develop ment of Ascidians shows that protostigmata are the bisected halves of tongue-barred U-shaped slits, as in Amplbioxus, and that lateral atria actually precede their formation. The possession by Appendicularians of a testless area behind an anterior test producing one, indicates that the genito-spiracular area is really a shallow atrio-cloacal cavity, like that of Doliolum, reduced and everted. The intestinal loop of Tunicates, adult or larval, is sinistral (i.e., rectum to left of oesophagus) except .in Doliolids and Appendicularians. Only in these and in one or two Salps is the loop straightened out : only in the same two groups are rectum and anus twisted to the right. The Appendicularian condition is thus the climax of a retrograde evolutional process of which the Thaliacean, especially the Doliolid, type, with posterior cloaca, reduced atria, and a few simple protostigmata (in the oozooid stage of development) is regarded as an intermediate step.