Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/754

 A R T H R O P O D A of the King-Crab (Limulus) is the only recognized suppressed. They are reciprocally exclusive. It seems instance of ommatidia in their simplest state. Each can not improbable that they are two modifications of the be readily compared with the single-layered lateral eye of same tissue-elements. In Peripatus the stigmatic pits at the Scorpion. In Crustacea and Hexapoda of all grades which the tracheae communicate with the atmosphere are we find compound eyes with the more complicated omma- scattered and not definite in their position. In other cases tidia described above. We do not find them in any the stigmata are definitely paired and placed in a few segments or in several. It seems that we have to suppose Arachnida. It is difficult in the absence of more detailed knowledge that the vasifactive tissue of Arthropoda can readily take as to the eyes of Chilopoda and Diplopoda to give full the form of air-holding instead of blood-holding tubes, and value to these facts in tracing the affinities of the various that this somewhat startling change in its character has classes of Arthropods. But they seem to point to a taken place independently in several instances—viz., in community of origin of Hexapods and Crustacea in regard the Onychophora, in more than one group of Arachto the complicated ommatidia of the compound eye, and nida, in Diplopoda, and, again, in the Hexapoda and to a certain isolation of the Arachnida, which are, however, Chilopoda. The Malpighian Tubes.—This name is applied to the traceable, so far as the eyes are concerned, to a distant common origin with Crustacea and Hexapoda through the numerous fine caecal tubes of noticeable length developed very simple compound eyes (monostichous, polymeniscous) from the proctodaeal invert of ectodermal origin in Hexapods. These tubes are shown to excrete nitrogenous waste of Limulus. The Tracheae.—In regard to tracheae the very natural products similar to uric acid. Tubes of renal excretory tendency of zoologists has been until lately to consider them function in a like position occur in most terrestrial Arthroas having once developed and once only, and therefore to poda—viz., in Chilopoda, Diplopoda, and Arachnida. They hold that a group “ Tracheata ” should be recognized, are also found in some of the semi-terrestrial and purely including all tracheate Arthropods. We are driven by aquatic Amphipod Crustaceans. But the conclusion that the conclusions arrived at as to the derivation of the all such tubes are identical in essential character seems to Arachnida from branchiate ancestors, independently of the be without foundation. The Malpighian tubes of Hexapods other tracheate Arthropods (see article Arachnida), to are outgrowths of the proctodseum, but those of Scorpion formulate the conclusion that tracheae have been inde- and the Amphipod Crustacea are part of the metenteron or pendently developed in the Arachnidan class. We are endodermal gut, though originating near its junction with also, by the isolation of Peripatus and the impossibility of the proctodseum. Hence the presence or absence of such tracing to it all other tracheate Arthropoda, or of regarding tubes cannot be used as an argument as to affinity withit as a degenerate offset from some one of the tracheate out some discrimination. The Scorpion’s so - called classes, forced to the conclusion that the tracheae of the Malpighian tubes are not the same organs as those so Onychophora have been independently acquired. Having named in the other Tracheata. Such renal csecal tubes accepted these two conclusions, we formulate the generaliza- seem to be readily evolved from either metenteron or tion that tracheae can be independently acquired by various proctodseum when the conditions of the out-wash of nitrobranches of Arthropod descent in adaptation to a terrestrial genous waste-products are changed by the transference as opposed to an aquatic mode of life. A great point of from aquatic to terrestrial life. The absence of such interest therefore exists in the knowledge of the structure renal cseca in Limulus and their presence in the terrestrial and embryology of tracheae in the different groups. It Arachnida is precisely on a parallel with their absence must be confessed that we have not such full knowledge in aquatic Crustacea and their presence in the feebly on this head as could be wished for. Tracheae are essen- branchiate Amphipoda. tially tubes like blood-vessels—apparently formed from We shall now pass the groups of the Arthropoda in the same tissue elements as blood-vessels—which contain review', attempting to characterize them in such a way as air in place of blood, and usually communicate by definite will indicate their probable affinities and genetic history. orifices, the tracheal stigmata, with the atmosphere. Sub-Phylum ARTHROPODA. — The characters of the subThey are lined internally by a cuticular deposit of chitin. phylum and those of the associated sub-phyla Chsetopoda and RotiIn Peripatus and the Diplopods they consist of bunches of fera, have been given above, as well as the general characters of fine tubes which do not branch but diverge from one the phylum Appendiculata which comprises these great sub-phyla. another ; the chitinous lining is smooth. In the Hexapods Grade A.—Hyparthropoda. and Chilopods, and the Arachnids (usually), they form treeHypothetical forms. like branching structures, and their finest branches are Grade B.—Protarthropoda. finer than any blood - capillary, actually in some cases The integument is covered by a delicate soft cuticle (not firm penetrating a single cell and supplying it with gaseous or (a) plated) which allows the body and its appendages great range of oxygen. In these forms the chitinous lining of the tubes extension and contraction. is thickened by a close-set spiral ridge similar to the spiral (&) The paired claws on the ends of the parapodia and the fangthickening of the cellulose wall of the spiral vessels of like modifications of these on the first post-oral appendages (manare the only hard chitinous portions of the integument. plants. It is a noteworthy fact that other tubes in these dibles) (c) The head is deuterognathous—that is to say, there is only same terrestrial Arthropoda—namely, the ducts of glands— one prosthomere, and accordingly the first and only pair of hemiare similarly strengthened by a chitinous cuticle, and that gnaths is developed by adaptation of the appendages of the second a spiral or annular thickening of the cuticle is developed somite. {d) The appendages of the third somite (second post-oral) are in them also. Chitin is not exclusively an ectodermal clawless oral papillae. product, but occurs also in cartilaginous skeletal plates {e) The rest of the somites cany equi-formal simple appendages, of mesoblastic origin (connective tissue). The immediate consisting of a conn or axis tipped with two chitinous claws and cavities or pits into which the tracheal stigmata open devoid of rami. The segmentation of the body is anomomeristic, there being appear to be in many cases ectodermic in sinkings, but no(/) fixed number of somites characterizing all the forms included. there seems to be no reason (based on embryological {g) The pair of eyes situated on the prosthomere are not of the observation) for regarding the tracheae as an ingrowth of Euarthropod type, but resemble those of Chsetopods (hence Nereidthe ectoderm. They appear, in fact, to be an air-holding ophthalmous). {h) The muscles of the body-wall and gut do not consist of transmodification of the vasifactive connective tissue. Tracheae versely-striped muscular fibre, but of the unstriped tissue observed are abundant just in proportion as blood-vessels become also in Chsetopoda. 698