90 SHELL GALLERY. 



eye in the Asciclian 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 Lar,vacea, 

 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. „ Compositse, 7 P'amilies. 



Sub-order 3. „ Sal pi formes, 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 Pyrosoma, they lead a fixed or stationary 

 life. 



Suh-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 Molgulidm 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. 



