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 A R T H R O P O D A front of the pectoral fins ; the branchiomeronies lose all relation to to the same law as the Arthropods. Intercalation of meromes, the position of the meromes of muscular, skeletal, ccelomic, and branchial, vertebral and dermal (fin-supports) seems to have taken nervous nature, and the heart and its vessels may move backwards place in Vertebrata in the fishes, while in higher groups intercalafrom their original metameres in higher Vertebrates carrying nerve- tion of vertebrae in large series has been accepted as the only possible explanation of the structural facts established by the comparison of loops with them. The Seventh Law of metamerism is one which has been pointed allied groups. The elucidation of this matter forms a very importout to the writer by Mr. E. S. Goodrich of Merton College, Oxford. ant part of the work lying to the hand of the investigator of It maybe called the law of “translation of heterosis.” Whilst vertebrate anatomy, and it is possible that the application of actual physical transference of the substance of meromes undeniably Goodrich’s law (the seventh of our list) may throw new light on takes place in such a case as tire passage of the pelvic fins of some the matter. In regard to the diminution in the number of somites in the fishes to the front of the pectorals, and in the case of the backward movement of the opisthosomatic appendages of spiders, yet the course of the historical development of those various groups more frequent mode in which an alteration in the position of a of metamerized animals, which have undoubtedly sprung specialized organ in the series or scale of metameres takes place is from ancestors with more numerous somites than they themnot by migration of the actual material organ from somite to somite, selves possess, it appears that we may formulate the following but by translation of the quality or morphogenetic peculiarity from laws as the tenth, eleventh, twelfth, and thirteenth laws of somite to somite accompanied by correlative change in all the metamerism. The Tenth Law is that individual somites tend to atrophy and somites of the series. The phenomenon may be compared to the transposition of a piece of music to a higher or lower key. It is finally disappear as distinct structures, most readily at the anterior thus that the lateral fins of fishes move up and down the scale of and the posterior ends of the series constituting an animal body. vertebral somites ; and thus that whole regions (tagmata), such as This is very generally exhibited in the head of Arthropoda, where, those indicated by the names cervical, thoracic, lumbar, and sacral, however, the operation of the law is largely modified by fusion (see are translated (accompanied by terminal increase or decrease in the below). With regard to the posterior end of the body, the atrophy total number of somites) so as to occupy differing numerical of segments does not, as a rule, affect the telson itself so much as positions in closely allied forms (cf. the varying number of cervical the somites in front of it and its power of producing new somites. Sometimes, however, the telson is very minute and nonchitinized somites in allied Reptiles and Birds). What, in this rapid enumeration, we will venture to call the (Hexapoda). The Eleventh Law may be stated thus: any somite in the Eighth Law of metamerism is the law of homoeosis, as it is termed by Bateson (1). Homoeosis is the making of a merome into the series which is the anterior or posterior somite of a tagma may likeness of one belonging to another metamere, and is the opposite become atrophied, reduced in size, or partially aborted by the of the process of “heterosis”—already mentioned. We cite this suppression of some of its meromes; and finally, such a somite law here because the result 'of its operation is to simulate the may disappear and leave no obvious trace in the adult structure occurrence of dislocation of meromes and has to be carefully dis- of its presence in ancestral forms. This is called the excalation tinguished from that process. A merome can and does in individual of a somite. Frequently, however, such “excalated” somites are cases of abnormality assume the form and character of the corre- obvious in the embryo or leave some merome (e.y., neuromere, sponding merome of a distant somite. Thus the antenna of an muscle, or chitin-plate) which can be detected by minute observainsect has been found to be replaced by a perfectly well-formed tion (microscopic) as evidence of their former existence. The walking leg. After destruction of the eye-stalk of a shrimp a new somite of the maxillipede (third post-oral appendage) of Apus growth appears, having the form of an antenna. Other cases are cancriformis is a good example of a somite on its way to excalafrequent in Crustacea, as individual abnormalities. They prove the tion. The third prse-oral and the prsemaxillary somites of Hexapod existence in the mechanism of metamerized animals, of structural insects are instances where the only traces of the vanished somite conditions which are capable of giving these results. What those are furnished by the microscopic study of early embryos. The structural conditions are is a matter for separate inquiry, which prsegenital somite of the Arachnida is an example of a somite we cannot even touch here. It is not improbable that homoeosis of which is preserved in some members of the group and partially or distant meromes may have given rise to permanent structural entirely excalated in other cases, sometimes with fusion of its changes characteristic of whole groups of Arthropoda, supposing remnants to neighbouring somites. The Twelfth Law of metamerism might very well be placed in the abnormality once established to be favoured by natural selection. Possibly the chelate condition of the prse-oral ap- logical order as the first. It is the law of lipoynerism, and asserts pendages of Arachnida may be due to homoeosis transferring the that just as the metameric condition is produced by a change in chelate form of post-oral limbs to what were previously antenni- the bodies of the descendants of unisegmental ancestors, so highly metamerized forms—i.e., strongly segmented forms with specialized form rami. We now come to the questions of the production of new somites regions of differentiated metameres — may gradually lose their or the addition of new somites to the series, and the converse prob- metamerized structure and become apparently and practically lem of the suppression of somites, whole or partial. We state as unisegmental animals. The change here contemplated is not the the Ninth Law of metamerism “that new somites or metameres atrophy of terminal segments one by one so as to reduce the size of are added to a chain consisting of two or more somites by growth the animal and leave it finally as a single somite. On the contrary, and gradual elaboration—what is called ‘ budding —of the anterior no loss of size or of high organization is necessary. But one by border of the hindermost somite. This hindermost somite is there- one, and gradually, the metameric grouping of the bodily structures fore different from all the other somites and is called the ‘telson.’ disappears. The cuticle ceases to be thickened in rings — the However long or short or heteromerized the chain may be, new muscles of the body-wall overrun their somite boundaries. Internal metameres or somites are only produced at the anterior border of septa disappear. The nerve - ganglia concentrate or else become the telson, except in the Vertebrata. ” That is the general law. But diffused equally along the cords ; one pair of renal ccelomoducts amongst some groups of metamerized animals partial exceptions and one pair of genital ccelomoducts grow to large size and remain to it occur. It is probably absolutely true for the Arthropoda —the rest disappear. The appendages atrophy or become limited from lowest to highest. It is not so certain that it is true for the to one or two pairs which are widely dislocated from their ancestral Ohsetopoda and would need modification in statement to meet the position. The animal ceases to present any indication of metameric cases of fissiparous multiplication occurring among Syllids and repetition of parts in its entire structure. Degrees in this process are Naidids. In the Vertebrata, where tagmosis and heterosis of frequently to be recognized. We certainly can observe such a meromes and dislocation of meromes and tagmata are, so to speak, change in the posterior region of some Arthropods, such as the rampant, new formation of metameres (at any rate as represented by hermit-crabs and the spiders. Admitting that the Echiurids are important meromes) takes place at more than one point in the chain. descended from Chsetopoda, such a change has taken place in them, Such points are found where two highly diverse “tagmata” abut amounting to little short of complete lipomerism, though not on one another. It is possible, though the evidence at present is absolutely complete. entirely against the supposition, that at such points in Arthropoda Recent suggestions as to the origin of the Mollusca involve the new somites may be formed.1 Such new somites are said to be supposition that such an effacement of once .well-marked meta“intercalated.” The question of the intercalation of vertebrae in merism has occurred in them, leaving its traces only in a few the Vertebrata has received some attention. It must be remembered structures such as the multiple gill-plumes and shell-shields of the that a vertebra even taken with its muscular, vascular and neural Chitons and the duplicated renal sacs of Nautilus. accessories is only a partial metamere—a merome, and that, so far A further matter of importance in this connexion is that when as complete metameres are concerned, the Vertebrata do conform the old metameres have been effaced a new secondary segmentation 1 The curious case of superabundant parapodia in the hinder in size, the number of extra pairs of legs on a non-terminal somite somites of Apus has already been cited and referred to as an example increases in number, these added meromes are certainly intercalated of autorhythmic multiplication of meromes. There is some reason for and represent incomplete intercalated metameres. The intercalation regarding the extra pairs of legs as being “ intercalated ” after the of new elements does not really go much further than this in Yerteformation of the somite as a single unit or merome by growth from the brata, for a vertebra with its myoskeletal tissues is only a merome and telson. Supposing, as appears to be the case, that as the Apus increases not a complete metamere. 692