Recent studies on the development of Echinoderms[61] have made it probable that the five body-cavities of Balanoglossus are represented in the larvae of those animals; and this materially strengthens the probability of the view that the respective adults are also allied.[62] It may be added that the relationship which appears to be indicated is between Balanoglossus and the bilateral ancestors from which the radially-symmetrical Echinoderms are probably descended. In comparing the Enteropneusta with the Pterobranchia, the disproportionate size of the trunk of Balanoglossus may perhaps be explained by assuming that the region of the third body-cavities has been enlarged since Balanoglossus branched off from the ancestral stock.[63] The approximation of the anus to the mouth in Pterobranchia is perhaps the result of their tubicolous habits.[64] In the position of the central nervous system in the skin of the collar, _Cephalodiscus_ appears to be more primitive {32}than Balanoglossus, as has been pointed out by Morgan.[65] It is not impossible that the presence of one pair of gill-slits in _Cephalodiscus_ indicates that this animal diverged from the ancestors of Balanoglossus before the gill-slits were metamerically repeated. ASCIDIANS AND AMPHIOXUS BY W. A. HERDMAN, D.SC. (EDINB.), F.R.S. Professor of Natural History in the University of Liverpool {35}CHAPTER II TUNICATA (ASCIDIANS AND THEIR ALLIES) INTRODUCTION—OUTLINE OF HISTORY—STRUCTURE OF A TYPICAL ASCIDIAN—EMBRYOLOGY AND LIFE-HISTORY The TUNICATA are marine animals found in practically all parts of the sea, and at all depths. They extend from the Arctic and Antarctic regions to the tropical waters, and from the littoral zone down to the abyssal depths of over three miles. They are abundant in British seas. They vary greatly in shape and colour, and range in size from an almost invisible hundredth of an inch to large masses a foot or more in diameter. And yet most Tunicata have a characteristic appearance by which they can be readily distinguished from other animals. They form a well-defined group, with definite anatomical characters, and there are no known forms intermediate between them and other groups. The Tunicata were formerly regarded as constituting, along with the Polyzoa and the Brachiopoda, the Invertebrate Class "MOLLUSCOIDEA." They are now known to be a degenerate branch of the lower CHORDATA, and to be more nearly related to the Vertebrata than to any group of Invertebrates.