530
ARACHNIDA
recognized in the memoir on the muscular and skeletal systems of Limulus and Scorpio by Lankester, Beck, and Bourne (4). 6. Alimentary Canal and Gastric Glands.—The alimentary canal in Scorpio, as in Limulus, is provided with a powerful suctorial pharynx, in the working of which extrinsic muscles take a part. The mouth is relatively smaller in Scorpio than in Limulus—in fact is minute, as it is in all the terrestrial Arachnida which suck the juices of either animals or plants. In both, the alimentary canal takes a straight course from the pharynx (which bends under it downwards and backwards towards the mouth in Limulus) to the anus, and is a simple, narrow, cylindrical tube (Fig. 33). The only point in which the gut of Limulus resembles that of Scorpio rather than that of any of the Crustacea, is in possessing more than a single pair of ducts or lateral outgrowths connected with ramified gastric glands or gastric caeca. Limulus has two pairs of these, Scorpio as many as six pairs. The Crustacea never have more than one pair. The minute microscopic structure of the gastric glands in the two animals is practically identical. The functions of these gastric diverticula have never been carefully investigated. It is very probable that in Scorpio they do not serve merely to secrete a digestive fluid (shown in other Arthropoda to resemble the pancreatic fluid), but that Fig. 33.—The alimentary canal and they also become distended by gastric glands of a Scorpion (A) e „ ..r Ql1„Vorl and of Limulus (B). ps, Muscular , JUlCes 01 lUe prey SUCKCtl suctorial enlargement of the in by the Scorpion as cer1 pharynx; sal, prosomatic pair of , • i , . . gastric caeca in Scorpio, called hcliniy must OCCUF in tile CftSC of the ^branched gasof gastric caeca and ducts4 of the5 trie CSBCa of the Spiders. mesosomatic region: c3 c , and c , rm„„ , • L , , -,.™ caeca and ducts of Scorpio not I he most important dlfterrepresented in Limulus; M, the ence which exists between the Malpighian or renal caecal diverti- , , ,. x . . . ,, cula of Scorpio ; pro, the procto- StmctUPG 01 Jjimulus cinci tilclt daeum or portion of gut leading to 0f gcornio is found in 10Una ln tbn me anus and formed embryologically . . by an inversion of the epibiast at hinder region of the alimentthat orifice. (From Lankester, „ct • • -i “ Limulus an Arachnid.”) canal. Scorpio is here provided with a single or double pair of renal excretory tubes, which have been identified by earlier authors with the Malpighian tubes of the Hexapod and Myriapod insects. Limulus is devoid of any such tubes. We shall revert to this subject below. 7. Ovaries and Spemnaries : Gonocoels and Gonoducts.— The scorpion is remarkable for having the specialized poition of coelom from the walls of which egg-cells or sperm-cells are developed according to sex, in the form of a simple but extensive network. It is not a pair of simple tubes, nor of dendriform tubes, but a closed net-work. The same fact is true of Limulus, as was shown by Owen (7) in regard to the ovary, and by Benham (14) in regard to the testis. This is a very definite and remarkable agieement, since such a reticular gonocoel is not found in Oi ustacea (except in the male Apus). ^Moreover, there is a significant agreement in the character of the spermatozoa of Limulus and Scorpio. The Crustacea are—with the exception of the Cirrhipedia—remarkable for having stiff,
motionless spermatozoids. In Limulus Lankester found (15) the spermatozoa to possess active flagelliform “tails,” and to resemble very closely those of Scorpio which, as are those of most terrestrial Arthropoda, are actively motile. This is a microscopic point of agreement, but is none the less significant. In regard to the important structures concerned with the fertilization of the egg, Limulus and Scorpio differ entirely from one another. The eggs of Limulus are fertilized in the sea after they have been laid. Scorpio, being a terrestrial animal, fertilizes by copulation. The male possesses elaborate copulatory structures of a chitinous nature, and the eggs are fertilized in the female without even quitting the place where they are formed on the wall of the reticular gonocoel. The female scorpion is viviparous, and the young are produced in a highly developed condition as fully formed scorpions. Differences between Limulus and Scorpio.—We have now passed in review the principal structural features in which Limulus agrees with Scorpio and differs from other Arthropoda. There remains for consideration the one important structural difference between the two animals. Limulus agrees with the majority of the Crustacea in being destitute of renal excretory caeca or tubes opening into the hinder part of the gut. Scorpio, on the other hand, in common with all air-breathing Arthropoda except Peripatus, possesses these tubules which are often called Malpighian tubes. A great deal has been made of this difference by some writers. It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcely been appreciated by the writers in question), really belongs to the Crustacean line of descent, whilst Scorpio, by possessing Malpighian tubes, is declared to be unmistakably tied together with the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes. It must be pointed out that the presence or absence of such renal excretory tubes opening into the intestine appears to be a question of adaptation to the changed physiological conditions of respiration, and not of morphological significance, since a pair of renal excretory tubes of this nature is found in certain Amphipod Crustacea (Talorchestia, &c.) which have abandoned a purely aquatic life. This view has been accepted and supported by Professors Korschelt and Heider (16). An important fact in its favour was discovered by Laurie (17), who investigated the embryology of two species of Scorpio under Lankester’s direction. It appears that the Malpighian tubes of Scorpio are developed from the mesenteron, viz., that portion of the gut which is formed by the hypoblast, whereas in Hexapod insects the similar ciecal tubes are developed from the proctodeum or in-pushed portion of the gut which is formed from epibiast. In fact it is not possible to maintain that the renal excretory tubes of the gut are of one common origin in the Arthropoda. They have appeared independently in connexion with a change in the excretion of nitrogenous waste in Arachnids, Crustacea, and the other classes of Arthropoda when aerial, as opposed to aquatic, respiration has been established—and they have been formed in some cases from the mesenteron, in other cases from the proctodseum. Their appearance in the air-breathing Arachnids does not separate those forms from the water-breathing Arachnids which are devoid of them, any more than does their appearance in certain Amphipoda separate those Crustaceans from the other members of the class. Further, it is pointed out by Korschelt and Heider that the hinder portion of the gut frequently acts in Arthropoda as an organ of nitrogenous excretion in the absence
