90 SHELL GALLEKY. 
eye in the Ascidian tadpole further tending to confirm the truth of 
this theory. Apart from a knowledge of the course of their develop- 
ment, Tunicata would have been classed among the Invertebrata, 
but the structure of the larva clearly reveals the affinities of the 
group to the backboned animals. 
Ascidia mentula belongs to the group of Simple Ascidians which 
are all fixed, and are either solitary or joined into colonies in which 
each individual or ascidiozooid has a distinct test of its own. In the 
Compound Ascidians, which form colonies by budding, the ascidio- 
zooids are buried in a common investing mass and have no separate 
tests. In a third group, the Salpa-like Ascidians, the ascidiozooids 
are united to form free-swimming colonies shaped like hollow cylinders 
open at one end. The above three groups belong to one great 
Order — the Ascidiacea. A second Order, Thaliacea, includes 
the free-swimming Salpa and Doliolum, which exhibit alternation 
of generations in their life history. A third Order Larvacea, 
includes very minute free-swimming forms which possess a tail in 
the adult stage. There are sixteen families of Tunicata. 
The following is a tabular view of Prof. Herdman's classifica- 
tions : — 
! Sub-order 1. Ascidise Simplices, 4 Families. 
Sub-order 2. „ Composite, 7 Families. 
Sub-order 3. „ Salpiformes, 1 Family. 
Order II. Thaliacea . . 3 Families. 
Order III. Larvacea ... 1 Family. 
Order I. — Ascidiacea. 
The Ascidiacea include the great majority of species. With the 
exception of the one genus Pt/rosoma, they lead a fixed or stationary 
life. 
Sub-order 1. — Ascidia Simplices. 
The Simple Ascidians are mostly solitary ; in a few forms, however, 
colonies arise by budding from stolons, but each individual has a dis- 
tinct test. The four families into which the sub-order is divided are 
chiefly characterised by the nature of the test, the number of lobes round 
the branchial and atrial orifice, and the character of the branchial sac. 
In the family Molgulidce, the tough membranous test is often 
coated with sand ; the branchial aperture is six-lobed, the atrial 
four-lobed, the branchial sac has long folds or pleats, and the 
stigmata are curved or arranged in spirals. 
