X .— The morphology and evolution of the ambulacrum in the echinoidea Holectypoida

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Abstract
During the course of the work upon which the following pages are primarily based, the need for an ever-increasing width of scope has continually arisen. In its original conception the paper was to treat of the structure of the Holectypoida alone. Gradually the inevitable comparisons extended their range, until in its final form the work deals more or less completely with the ambulacral structure of all the orders of the Echinoidea, and includes far more argument of a general type than was first intended. Nevertheless, the original title has been retained. As in a map designed to show the position of a town, a large area of the surrounding district must needs be included, so that the roads which lead to, from, or past the place can be distinguished; thus, too, the relative position of the Holectypoid ambulacrum in the morphogenesis of Echinoid ambulacra can only be appreciated by description of the homologous structures which preceded, succeeded, or diverged from it in the course of evolution. Thus the title, though far from comprehending all the matter subjoined to it, is apt, in that it indicates the central topic towards which all the others converge and contribute. In the study of the Echinoidea, the ambulacra and their associated structures have always been recognised as affording taxonomic evidence of great value. Not only do the "avenues” of pores attract attention, in denuded tests, by their distinctness and diversity, but the extensions of the water-vascular system to which they give passage are concerned with many vital functions. Ambulacral plates are among the first to appear during the metamorphosis of an Echinoid pluteus, and the development of ambulacral projections from the hydrocoel marks an early stage in larval life. An ambulacrum consists of an even number of columns of plates (usually two), each one of which is perforated for the transmission of a “tube-foot.’' There are thus two features, more or less interdependent, that are available for study in fossil forms—the plates and the pores. But it is only within the last five decades that any consistent attempts at an understanding of the plating-structure of the Echinoid corona have been made; and even now systematic writers most frequently limit their descriptions of the ambulacra to the nature of the pores and the distribution of the ornament.

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