↑ | Molgulidae | Ascidiidae | +––––––––Amphioxus (5) | | P | +–––––––+–––––+–––––+––––––+––– Cynthiidae. r | | | | o | +– C | | t | Larvacea. | l | (6) Polystyelidae o | | | a | c | (1) (2) | +– v Botryllidae h +––––––––––––––+––––––––––––| e o | Proto–Tunicata. | l r | +– i Distomatidae d | Doliolidae–+ | n | a | | | i | Didemnidae and t | | | d | | Diplosomatidae a | | | a | | | | (3)| e (4) | | | +–––––––+–––––––+––––+––––––––––+––––Polyclinidae | Salpidae–––+ | | Pyrosomatidae The Tunicata are remarkable for the variety in appearance, structure, and life-history which they present. No group illustrates in a more instructive manner so large a number of important biological principles and phenomena. They show solitary and colonial forms, fixed and free, pelagic and abyssal. The development is in some cases larval and with metamorphosis, in others abbreviated and direct. Persistent traces of ancestral characters are seen in the embryonic and larval stages, while the adults present the most varied secondary adaptations to littoral, {111}pelagic, and deep-sea, free-swimming and sessile modes of existence. In the details of their classification they demonstrate both stable and variable species, monophyletic and polyphyletic groups. They exhibit the phenomena of gemmation and of embryonic fission, of polymorphism, hibernation, alternation of generations, and change of function. They have long been known as a stock example of degeneration; but in fact they lend themselves admirably to the exposition of more than one "Chapter of Darwinism." * * * * * NOTE TO P. 78.—_Oligotrema_, Bourne (_Quart. J. Micr. Sci_. xlvii. Pt. ii. 1903, p. 233), a Molgulid from the Loyalty Islands, has a reduced branchial sac and greatly developed pinnate, muscular branchial lobes, probably used in capturing food. {112}CHAPTER IV CEPHALOCHORDATA INTRODUCTION—GENERAL CHARACTERS—ANATOMY OF AMPHIOXUS—EMBRYOLOGY AND LIFE-HISTORY—CLASSIFICATION OF CEPHALOCHORDATA—SPECIES AND DISTRIBUTION