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LECTURE III.

ON THE CLASSIFICATION OF ANIMALS.

THE ECHINODERMATA, SCOLECIDA, ANNELIDA, CRUSTACEA, ARACHNIDA, MYRIAPODA, AND INSECTA.

HITHERTO, it has not been a matter of very great difficulty to discover the characters in which the members of the various classes, which have passed under our notice, agree with one another and differ from the members of all other classes. But to-day we shall be met, at the outset of our studies, by a large series of organisms which present us with much greater obstacles,—the result, in a great measure, of imperfect knowledge.

The first group on the list-the ECHINODERMATA-comprises the star-fishes, sea-urchins, sea-cucumbers, trepangs, and feather-stars-known technically as Asteridea, Echinidea, Holothuridea, Ophiuridea, Crinoidea, &c.,-marine animals which differ vastly in external appearance, though they all, in the adult state, present a more or less definitely radiate arrangement of some parts of their organization.

That which most remarkably distinguishes the Echinodermata is the nature of the embryo, and the strange character of the process by which the adult form is originated by a secondary development within that embryo.

In the great majority* of the Echinodermata, the develop

*In Ophiolepis squamata and Echinaster sepositus, the larva appears to attain only a very imperfect state of development before the appearance of the echinoderm body; and careful re-examination is required to decide how far the larvae of these animals are truly bilaterally symmetrical.

ment of which has been examined, the impregnated egg gives rise to a free-swimming, ovoid, ciliated embryo, the cilia of which soon become restricted to, and, at the same time, largely developed upon, one, two, or more bands, which are disposed either transversely, or more or less obliquely to the longitudinal axis of the body, but which are, in any case, bilaterally symmetrical (Fig. 17).

Fig. 17.

Fig. 17.-Diagram exhibiting the general plan of the development of the Echinoderms (after Müller).-A. Common form whence the Holothurid (B, B') and Ophiurid or Echinid (C, C') larvæ are derived. D, D'. Younger and more advanced stages of the Asterid (Bipinnaria) larvæ. a'. Mouth. b. Stomach. c, Intestine. d. Anus.

e. Ciliated band. e'. Second or anterior ciliated circlet.

The parts of the body which carry the ciliated band, or bands, often become developed into processes, which correspond upon each side of the body, and thus render its bilateral symmetry more marked (Fig. 17, C', D'). And, in the larvæ of some Echinidea and Ophiuridea, other bilaterally symmetrical processes are developed from parts of the body which do not lie in the course of the ciliated bands.

The larvæ of Asteridea and Holothuridea are devoid of any continuous skeleton, but those of Ophiuridea and Echinidea possess a very remarkable bilaterally symmetrical, continuous, calcareous skeleton, which extends into, and supports the processes of the body (Fig. 20).

A distinctly defined alimentary canal early makes its appearance in these Echinoderm larvæ. It is divided into a wellmarked oral and oesophageal portion, a globular stomach, and a short intestine terminating in an anal aperture (Figs. 17 and 18). All the parts of the alimentary canal are disposed in a longitudinal and vertical plane, dividing the larval body into two symmetrical halves; but the oesophageal and intestinal portions are so disposed as to make an angle, open towards the ventral side, with one another. No nervous, or other organs, besides those indicated, have as yet been discovered in these larvæ.

Fig. 18.-A young Asterid larva (after Müller).-A. Ventral. B. Lateral views of the larva. C. Commencing rudiment of the starfish. a. Mouth. b. Esophagus. c. Stomach. c'. Intestine. o. Anus. x. Anterior, and y, principal ciliated band. h. Cæcal diverticulum, forming the rudiment of the ambulacral vascular system, and opening externally by the pore, g. k. Perisoma of the starfish.

Fig. 19.-Development of a Holothurid (after Müller).-A. Early condition of larva.

B, C. Later stages. f, g, h, the ambulacral vascular system.

Fig. 20.-Development of an Echinid larva (after Müller).-A. earliest, and B, later condition of larva. C. The Echinid imago developed within and nearly obliterating the larva.

After swimming about in this condition for a while, the larva begins to show the first signs of those changes by which it is converted into the adult Echinoderm. An involution of the integument takes place upon one side of the dorsal region of the body, so as to give rise to a cæcal tube, which gradually elongates inwards, and eventually reaches a mass of formative matter, or blastema, aggregated upon one side of the stomach. Within this, the end of the tube becomes converted into a circular vessel, from which trunks pass off, radially, through the enlarging blastema. The latter, gradually expanding, gives rise in the Echinidea, the Asteridea, the Ophiuridea, and the Crinoidea, to the body-wall of the adult; the larval body and skeleton (when the latter exists), with more or less of the primitive intestine, being either cast off as a whole, or disappearing, or becoming incorporated with the secondary development, while a new mouth is developed in the centre of the ring formed by the circular vessel. The vessels which radiate from the latter give off diverticula to communicate with the cavities of numerous processes of the body-the so called feet-which are the chief locomotive organs of the adult. The radiating and circular vessels, with all their appendages, constitute what is known as the "ambulacral system;" and, in Asterids and Echinids, this remarkable system of vessels remains in communication with the exterior of the body by canals, connected with perforated portions of the external skeleton-the so-called madreporic canals " and " tubercles." In Ophiurids the persistence of any such communication of the ambulacral system

with the exterior is doubtful, and still more so in Crinoids. In Holothurids no such communication obtains, the madreporie canals and their tubercles depending freely from the circular canal into the perivisceral cavity.

Whether the larva possessed a skeleton or not, the adult Echinoderm presents a calcareous framework which is developed quite independently of that of the larva. This skeleton may be composed of mere detached spicula, or plates, as in the Holothurids; or of definitely disposed ossicula, or regular plates, as in other Echinoderms. In the latter case its parts are always disposed with a certain reference to the disposition of the ambulacral system, and hence have a more or less distinctly radiate arrangement. It might be expected, in fact, that the arrangement of the organs of support should follow more or less closely that of the chief organs of movement of the adult Echinoderm, and it is not surprising to find the nervous system similarly related. It is, in all adult Echinoderms, a ring-like, or polygonal, ganglionated cord, situated superficially to that part of the ambulacral system which surrounds the mouth, and sending prolongations parallel with, and superficial to, the radiating ambulacral trunks.

The reproductive organs of the Echinoderms, which usually open upon, or between, parts of the radially disposed skeleton, commonly partake of the radial symmetry of that skeleton; but they have no such radial symmetry in the Holothuridea.

The alimentary canal of the adult Echinoderm is still less dependent upon the skeleton, and only in one group, the Asteridea, exhibits anything approaching a radiate disposition. Where skeletal elements are developed around the mouth or gullet, however, they have a radial disposition; as, e. g., the parts of the so-called "lantern of Aristotle."

The vascular system which exists in many, if not all, adult Echinoderms, but the true nature of which is by no means understood at present, is closely related both to the alimentary and to the ambulacral systems, and partakes of the disposition

of both.

No Echinoderm whatsoever has its organs, internal or external, disposed with that absolute and perfect radial symmetry