Page:EB1911 - Volume 01.djvu/938

 and in the constitution of its digestive, vascular, respiratory (branchial), excretory, skeletal, nervous and muscular systems it exhibits what appears to be a primordial condition of vertebrate organization, a condition which is, in fact, partly recapitulated in the course of the embryonic stages of craniate vertebrates. In comparative morphology it provides many illustrations of important biological principles (such, for example, as substitution and change of function of organs), and throws new light upon, or at least points the way to new ideas of, the primitive relations of different organic systems in respect of their function and topography. One of the most puzzling features in its structure, and, at the same time, one of the greatest obstacles to the view that it is essentially primitive and not merely a degenerate creature, is the entire absence of the paired organs of special sense, olfactory, optic and auditory, which are so characteristic of the higher vertebrates. Although it is true that there is a certain amount of gradation in the degree of development to which these organs have attained in the various orders, yet it is hardly sufficient to enable the imagination to bridge over the gap which separates Amphioxus from the lowest fishes in regard to this feature of organization.

Classification.—On account of the absence of anything in the nature of a skull, Amphioxus has been regarded as the type of a division, Acrania, in contrast with the Craniata which comprise all the higher Chordata. The ordinal name for the genera and species of Amphioxus is Cephalochorda, the term referring to the extension of the primary backbone or notochord to the anterior extremity of the body; the family name is Branchiostomidae. The amount of generic divergence exhibited by the members of this family is not great in the mass, but is of singular interest in detail. There are two principal genera—1. Branchiostoma Costa, having paired sexual organs (gonadic pouches); 2. Heteropleuron Kirkaldy, with unilateral gonads. Of these, the former includes two subgenera, Amphioxus (s. str.) Yarrell and Dolichorhynchus Willey. The species belonging to the genus Heteropleuron are divided among the three subgenera, Paramphioxus Haeckel, Epigonichthys Peters, and Asymmetron Andrews. The generic characters are based upon definite modifications of form which affect the entire facies of the animals, while the specific diagnoses depend upon minor characters, such as the number of myotomes or muscle-segments.

Habits and Distribution.—With regard to its habits, all that need be said here is that while Amphioxus is an expert swimmer when occasion requires, yet it spends most of its time burrowing in the sand, in which, when at rest, it lies buried with head protruding and mouth wide agape. Its food consists of microscopic organisms and organic particles; these are drawn into the mouth together with currents of water induced by the action of the vibratile cilia which are abundant along special tracts on the sides and roof of the vestibule of the mouth and in the walls of the perforated pharynx (“ciliary ingestion”). Amphioxus favours a littoral habitat, and rarely if ever descends below the 50-fathom line. Species occur in all seas of the temperate, tropical and subtropical zones. The European species, A. lanceolatus, is found in the Black and Mediterranean Seas, and on the coasts of France, Great Britain and Scandinavia, while a closely allied species or subspecies, A. caribaeus, frequents the Caribbean region from Chesapeake to La Plata. A. californiensis occurs on the coast of California, and A. belcheri extends its area of distribution from Queensland through Singapore to Japan. A recently described species, Dolichorhynchus indicus, characterized by the great length of the praeoral lobe or snout, has been dredged in the Indian Ocean. Paramphioxus bassanus occurs on the coast of Australia from Port Phillip to Port Jackson; P. cingalensis at Ceylon. Epigonichthys cultellus (fig. 1) inhabits Torres Strait, and has also been found at Ternate. Asymmetron lucayanum is the Bahaman representative of the family, with a subspecies, A. caudatum, in the South Pacific from New Guinea to the Loyalty Islands. The Peruvian species, Branchiostoma elongatum, with nearly eighty myotomes, cannot at present be assigned to its proper subgenus.