the water. In striking contrast with it are the sluggish sea-urchins, whose protecting spines serve both for progression and defence, while the numerous cirri protruded from the openings in the ambulacral plates, acting on the principle of the sucker, enable the animal to anchor itself, or when occasion requires, to move up a perpendicular surface.

The Holothurias, while preserving the same general radiate type in certain organs, differ in this respect that their body is elongated-approaching the vermiform— and the integuments are generally soft. They present us with another modification adapting them for a dif ferent mode of life. Now moving by the suckers, which protrude from the pores of the skin, and again by the extension and contraction of their soft bodies, they are fitted for localities inaccessible to their allies, the starfishes and sea-urchins.

CHAPTER IX.

NERVOUS, VASCULAR, AND MUSCULAR SYSTEMS.

IN these systems of parts so essential to the animal economy, we may expect also to find examples of types and special adaptations, and our argument would be incomplete without some reference to the subject. It must not be supposed that the brevity with which we discuss this department is any indication of its inferior importance. More space has been devoted to types and modifications in the internal and external skeleton and appendages, because we believe that the proofs are more easily accessible to the general reader.

NERVOUS SYSTEM.

The presence of a system of nerves is the most marked character which separates the animal from the vegetable kingdom. In some of the lower forms, its existence has not been clearly demonstrated; in many it is very rudimentary. But as we rise higher in the scale we find an evident advance, commensurate with the endowments of the animal.

The simplest function of this system is that of conveying an impression sufficient to excite the contraction of muscular tissue, and thus effect some motion in an organ or its parts. The impression is conveyed by one set of nervous fibres to a centre--a ganglion, and from this it

is communicated to the muscle, which is thus stimulated to contract. This reflex function is not necessarily accompanied by sensation, and the movements of the lower forms of animal creation appear to be of the nature just mentioned. But when we take a general view of the animal kingdom, we find other superadded functions dependent on this system; "it is the instrument of consensual and instinctive actions, of mental processes, and of voluntary movements."*

In Mollusca, the typical nervous system is usually described as consisting of three sets of nervous centres or ganglia :—1st, cephalic, supplying the eyes and other parts about the head and mouth; 2d, pedal, supplying principally the foot; 3d, parieto-splanchnic, supplying the walls of the body, the heart and gills, &c. (See Fig. 45, parts marked n.) Now, we observe modifications of this type corresponding to the development of the dif ferent organs, and the necessities of the animal. In different mollusca, where the foot is more or less developed, we observe corresponding development of the pedal ganglion.

In the Cephalopoda, or cuttle-fishes, the large organs of vision, the complicated buccal apparatus, and active movements, are all in relation to the increase of nervous matter, and concentration of its parts. Professor Sharpey has further shewn an interesting modification in the nerves of the arms in evident harmony with the habits. of these cuttle-fishes. Each sucking disc (on the arms) is connected by a set of fibres with a ganglionic centre, while all the ganglia are at the same time brought into connexion by another fibrous tract with the cephalic portion of the nervous system. Each sucker can, therefore-by reflex action-attach itself to any body which

*Carpenter's Comparative Physiology.

touches it, while all are also under the control of the animal.

In Articulata, the typical nervous system consists of two nervous cords running parallel to each other, and connected at intervals by dilatations or ganglia in pairs. (See Fig. 67.) The general arrangement is such that every part of the body is furnished with two sets of nervous connexions; one of these is with the ganglion of its own segment, and another with the cephalic ganglia. The distribution of the nervous system in Articulata was an obvious relation to the general arrangement of the hard parts, the body, as we have seen, being composed of homotypal rings, bearing lateral appendages in pairs. And as we find various modifications of this type in harmony with some important function, we also find corresponding modifications in the nervous system. The late Mr. Newport, to whose investigations we owe so much in connexion with this subject, has shewn that in certain cases there is an enlargement of a portion of the nervous system at certain points, " corresponding to the apparent greater necessity for accumulations of nervous matter at those parts of the cord." This remark is generally applicable as regards the ganglia of the head, the arrangement being evidently in direct relation to the functions of the important appendages of that part. There are, farther, certain local modifications, having more special connexion with the appendicular organs. Mr. Newport states, regarding the nerves which supply the mandibles and maxillæ, that "union of those nerves at their base is interesting from the circumstance that during manducation a consentaneous movement of the parts is required, since, while the mandibles are employed in chewing, the maxillæ are also employed in turning and assisting to pass the food into the pharynx."

1

The concentration of the nervous matter in the thorax is evidently a modification of the type in conformity with the presence of wings and legs, the active appendages of that part of the body. The wings require the exercise of great mus- 2 cular effort in order to support the insect 8 during flight, and the distribution of the nervous matter is in accordance with this necessity. But there must also be perfect unison in their action, and this is also provided for. Mr. Newport has demonstrated that there is a remarkable peculiarity in the relations of the thoracic ganglia and their connecting fibres, in those insects in which both pairs of wings are actively con

FIG. 67.*

cerned in flight. He remarks, "that this is the reason for this curious union of the nerves for the wings, seems apparent from the circumstance that it exists in very many tetrapterous insects of rapid or powerful flight, as in the

FIG. 67. Nervous system of pupa of Sphinx ligustri, composed of two parallel nervous cords, for the most part joined together side by side, and connected knots of nervous matter-ganglions. The two larger masses and branches of nervous matter, or the upper part, supply organs in the head, as eyes, jaws, &c., viz., 1 and 2; the two nervous masses and branches succeeding to these supply the wings and legs, 8 and 4. The remaining portions are more uniform.