Imágenes de páginas
PDF
EPUB

eaters; and these two groups are nearly equivalent to Cuvier's two Orders, Dorsibranchiata and Tubicola.

The 2nd and 3rd Orders, Gymnocopa and Onycophora, are formed merely for receiving a few aberrant species of Annelides, such as the Tomopterus and Peripatus.

The 4th Order, Oligochata, contains the earth-worms or Lumbrici, and Naides, etc.

The 5th Order, Discophora, contains the leeches and their allies (Hirudo, etc.), and these two last Orders are nearly equivalent to the Abranchiata of Cuvier.

Grube is followed in this arrangement by Dr. Johnston, who gave the first impulse to the study of the Annelides in this country, and who by his writings has done more than any other man to popularize the study of these animals in Great Britain.

MM. Audouin and Milne Edwards have adopted a more natural method of arrangement. They divide the Annelides into four Orders

The 1st is the Errantia, or wandering Annelides, containing the great proportion of the free-moving worms, as the sea-mice (the Aphrodiseans), the Amphinomæ, Eunicæ, the Nereides, the Ariciæ, along with the Peripatus, the Chætopterus and Arenicolæ, or sandworms. This order is nearly equivalent to Cuvier's Dorsibranchiata.

The 2nd Order is that of the Tubicola, the tubicolous worms, which he also calls Sedentaria, or sedentary worms, containing the Serpulæ, Terebellæ, etc., and which is almost equivalent to the Tubicola of Cuvier.

The 3rd Order is that of the Terricola, or worms which live in the earth, such as the earth-worms (Lumbrici, etc.); and

The 4th is that of the Suctoria, or sucking-worms, containing the leeches (Hirudo) and their allies. These two Orders are equivalent to Cuvier's Abranchiata.

M. Quatrefages defines an Annelide to be an animal composed of rings or joints, on which the appendages repeat themselves throughout the body. According to this idea, he divides the true Annelides into two great Orders, according as this law of repetition of its members is more or less perfect. In the first Order, where this repetition of parts is perfect, we find the same parts repeated from one extremity of the body to the other. This Order is equivalent to the Errantia of Milne Edwards. In the second Order this law of repetition is not perfect; it is interrupted in several places, and the species contained in this Order correspond almost entirely with those of Milne Edwards's Tubicola, or Sedentaria. The

Terricola, Suctoria, and Gephyrea, he thinks ought to constitute distinct Orders, separated from the true Annelides.

The same law of repetition may perhaps be seen in the habits and manners of these creatures. The actions of these little humble beings,” says Mr. Williams, “are uniform and unvarying, for ever repetitions; the different individuals of the same species executing precisely the same movements when the circumstances are the same. The Sabellæ, in the construction of their tubes, repeat the same invariable 'round' of actions; they obey an impulsive principle which discovers no change of plan. The Terebellæ gather the shell fragments for the manufacture of their tubes on principles of the same monotonous uniformity. The sand-lug undermines the strand, generation after generation, with exact and undeviating regularity.

It would be impossible within the limits of this paper to follow the different methods proposed for the arrangement of the various individuals of the Annelides by such distinguished men as De Blainville, Savigny, and Lamarck, amongst the earlier naturalists, or by Schmarda, Ehlers, and other more recent authors; but as the arrangement of MM. Audouin and Milne Edwards appears to be a very natural one, and one which can easily be understood, I propose following it, only premising that as a fifth Order I add the Siphunculoid worms, or Gephyrea, which, as previously stated, are now by the latest authors, in accordance with the distribution of the nervous system, and other characters common to these worms and the other Annelides, included amongst the Annelidan worms.

I will now proceed to give a slight sketch of the anatomy of these animals.

Nervous System.--The general nervous system, in all Annelides, consists of a longitudinal series of ganglia, which extend throughout the whole length of the body. In the upper part, or the head, we see the cerebral ganglia or brain, called by Huxley the“ præ-esophageal” (Plate I., Figs. 1, 2, 3, a). From these ganglia nerves are sent off to the different parts near the head, the tentacles, antennæ, etc. (Plate I., Fig. 1, d, d). Lower down we see the longitudinal ganglionic chain, or as Huxley terms them, “post-esophageal ganglia," extending throughout the whole length of the body (Plate I., Fig. 1, c, c, Figs. 2, 3, d, d). A ganglion exists on each side, in every ring or segment of the body, and from these ganglia nerves are distributed to the feet and organs connected with them. From the cerebral ganglia, nervous branches or cords arise, which connect this portion of the nervous system with the ventral chain of

ganglia. These are always distinct, and constitute a closed circle, which is generally called the esophageal ring or collar, or as Quatrefages would prefer calling it, the buccal collar (Plate I., Figs. 1, 2, 3, 1,6). This distribution of the nervous system varies considerably in appearance in the different groups of the Annelides, but as a general rule, may be seen in all. In our Plate (I.) we give representations from Quatrefages of the nervous system in three of the Orders of Annelidan worms, which will show the arrangement as common to them all. Fig. 1 shows the nervous system of a Nereid, Fig. 2 that of a Serpula, and Fig. 3 that of an earth-worm (Lumbricus).

The cerebral or præ-esophageal ganglia are so placed that the esophagus lies upon or above them, whilst the visceral ganglia are so situated that the digestive canal or intestine lies underneath them, the esophagus having passed through, or been encircled by the nervous cords forming the esophageal ring or buccal collar. Mr. Williams thus states the different actions of these two great ganglia:— "The cephalic ganglia,” he says, “representing in situation and importance the brain of the higher articulata, is distinguished from the rest. It originates nerves to the organs of sense. "It is gifted, like a brain, with the distinctive power of directing, controlling, and co-ordinating the movements of the entire body; whilst the influence of each ganglion of the trunk is confined to its own segment."

Circulation of the Blood.--As Professor Huxley, in his lectures on Comparative Anatomy, says, “No Annelide ever possesses a heart comparable to the heart of a crustacean or an insect; but a system of vessels, with more or less extensively contractile walls, containing a clear fluid, usually red or green in colour, is very generally developed, and sends prolongations into the respiratory organs, when such exist." The vessels in which the blood flows are closed vessels, and generally consist of two main trunks, the dorsal (Plate I., Fig. 6, e, e) and the ventral (Fig. 6, f). The dorsal vessel carries the blood from the lower to the upper part of the body, whilst the ventral conveys the blood back from the upper to the lower extremities. At every segment of the body the dorsal vessel gives off branches—the number and disposition of which vary considerably—which distribute themselves upon the peripheric parts as well as the internal organs, whilst corresponding vessels return the blood from these to the ventral vessel. These secondary trunks, again, give off numerous small branches which terminate in a fine and close-set branchial net-work (see Plate I., Fig. 6).

[ocr errors][ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small][ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors]
« AnteriorContinuar »