- Modern views as to the classification and affinities of the Arachnida have been determined by the demonstration that Limulus and the extinct Eurypterines (Pterygotus, &c.) are Arachnida; that is to say, are identical in the structure and relation of so many important parts with Scorpio, whilst differing in those respects from other Arthropoda, that it is impossible to suppose that the identity is due to homoplasy or convergence, and the conclusion must be accepted that the resemblances arise from close genetic relationship. The view that Limulus, the king-crab, is an Arachnid was maintained as long ago as 182 9 by Strauss-Diirckheim (1), on the ground of its possession of an internal cartilaginous sternum - also possessed by the Arachnida (see figs.
The evidence of the exact equivalence of the segmentation and appendages of Limulus and Scorpio, and of a number of remarkable points of agreement in structure, was furnished by Ray Lankester in an article published in 1881 (" Limulus an Arachnid," Quart.
The various comparisons previously made between the structure of Limulus and the Eurypterines on the one hand, and that of a typical Arachnid, such as Scorpio, on the other, had been vitiated by erroneous notions as to the origin of the nerves supplying the anterior appendages of Limulus (which were finally removed by Alphonse Milne-Edwards in his beautiful memoir (6) on the structure of that animal), and secondly by the erroneous identification of the double sternal plates of Limulus, called " chilaria," by Owen, with a pair of appendages (7).
Once the identity of the chilaria with the pentagonal sternal plate of the scorpion is recognized - an identification first insisted on by Lankester - the whole series of segments and appendages in the two animals, Limulus and Scorpio, are seen to correspond most closely, segment for segment, with one another (see figs.
The structure of the prosomatic appendages or legs is also seen to present many significant points of agreement (see figures), but a curious discrepancy existed in the six-jointed structure of the limb in Limulus, which differed from the seven-jointed limb of Scorpio by the defect of one joint.
Pocock of the British Museum has observed that in Limulus a marking exists on the fourth joint, which apparently indicates a previous division of this segment into two, and thus establishes the agreement of Limulus and Scorpio in this small feature of the number of segments in the legs (see fig.
The tergites, or chitinized dorsal halves of the body rings, are fused to form a " prosomatic carapace," or carapace of the prosoma, in both Limulus and Scorpio (see figs, 7 and 8).
On the surface of the carapace there are in both animals a pair of central eyes with simple lens and a pair of lateral eyetracts, which in Limulus consist of closely-aggregated simple eyes, forming a " compound" eye, whilst in Scorpio they present several AC separate small eyes.
Differ; but the lateral eyes of Scorpio were shown by them (After Lankester, loc. cit.) to be similar in structure to the lateral eyes of Limulus, and the central eyes of Scorpio to be identical in structure with the central eyes of Limulus (see below).
The second pair of mesosomatic appendages in Scorpio are known as the " pectens."
The next four pairs of appendages (completing the mesosomatic series of six) consist, in both Scorpio and Limulus, of a base carrying each 130 to 150 blood-holding, leaf-like plates, lying on one another like the leaves of a book.
The difference between the gill-books of Limulus and the lung-books of Scorpio depends on the fact that the latter are adapted to aerial respiration, while the former serve for aquatic respiration.
The appendage carrying the gill-book stands out on the surface of the body in Limulus, and has other portions developed besides the gill-book and its base; it is fused with its fellow of the opposite side On the other hand, in Scorpio, the gill-book-bearing apFIG.
That this (Alter Lankester, loc. cit.) in-sinking has taken place, and that the lung-books or in-sunken gill-books of Scorpio really represent appendages (that is to say, limbs or parapodia) is proved by their developmental history (see FIG.
15); but we are yet in need of evidence as to the exact equivalence of margins, axis, &c., obtaining between the lung-book of Scorpio and the gill-book of Limulus.
The identification of the lung-books of Scorpio with the gill-books of Limulus is practically settled by the existence of the pectens in Scorpio (fig.
The elongated axis which opens at the stigma in Scorpio and which can be cleared of soft, surrounding tissues and co agulated blood so as to present the appearance of a limb axis carrying the book-like leaves of the lung is not really, as it would seem to be at first sight, the limb axis.
Passing on now from the mesosoma we come in Scorpio to the metasoma of six segments, the first of which is broad whilst the rest are cylindrical.
Following the metasoma in Limulus, we have as in Scorpio the post-anal spine - in this case not a sting, but a powerful and important organ of locomotion, serving to turn the animal over when it has fallen upon its back.
Perhaps the most important general agreement of Scorpio compared with Limulus and the Eurypterines is the division of the body into the three regions (or tagmata) - prosoma, mesosoma and metasoma - each consisting of six segments, the prosoma having leg-like appendages, the mesosoma having foliaceous appendages, and the metasoma being destitute of appendages.
In 1893, some years after the identification of the somites of Limulus with those of Scorpio, thus indicated, had been published, zoologists were startled by the discovery by a Japanese zoologist, Kishinouye (8), of a seventh prosomatic somite in the embryo of Limulus longispina.
The simple identification of somite with somite in Limulus and Scorpio seemed to be threatened by this discovery.
But in 1896 Dr August Brauer of Marburg (9) discovered in the embryo of Scorpio a seventh prosomatic somite (see VII PrG, figs.
In the case of Scorpio this segment is indicated in the embryo by the presence of a pair of rudimentary appendages, carried by a well-marked somite.
As in Limulus, so in Scorpio, this unexpected somite and its appendages disappear in the course of development.
Owing to its position it is convenient to term the somite which is excalated in Limulus and Scorpio " the praegenital somite."
(After Lankester, loc. cit.) the last pair of legs in both Scorpio and Limulus, viz.
The pentagonal sternite of Scorpio (fig.
This interpretation, however, of the " metasternites " of Limulus and Scorpio is opposed by the coexistence in Thelyphonus (figs.
The first segment of the mesosoma of Scorpio and Limulus thus remains the first segment, and can be identified as such throughout the Eu-arachnida, carrying as it always does the genital apertures.
There are a number of other important points of structure besides those referring to the somites and appendages in which Limulus agrees with Scorpio or other Arachnida and differs from other Arthro- '11'1 poda.
The cerebral mass is in Limulus more easily separated by dissection as a median lobe distinct from the laterally placed ganglia of the cheliceral somite than is the case in Scorpio, but the relations are practically the same in the two forms. Formerly it was supposed that in Limulus both the chelicerae and the next following pair of appendages were prosthomerous, as in Crustacea, but the dissections of Alphonse Milne-Edwards (6) demonstrated VI FIG.
Limulus thus agrees with Scorpio and differs from the Crustacea, in which there are three prosthomeres - one ocular and two carrying palpiform appendages.
- The prosomatic appendages of Limulus polyphemus (right) and Scorpio (left), Palamnaeus indus compared.
The central nervous systems of Limulus and of Scorpio present closer agreement in structure than can be found when a Crustacean is compared with either.
(See Patten (42) for important observations on the neuromeres, &c., of Limulus and Scorpio.) 2.
- Limulus agrees with Scorpio not only in having a pair of central eyes and also lateral eyes, but in the microscopic structure of those organs, which differs in the central and lateral eyes respectively.
The lateral eyes of Scorpio consist of groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus.
The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus.
23) each ommatidium has a peculiar ganglion cell developed in a central position, whilst the ommatidium of the lateral eyelets of Scorpio shows!
The struc ture of the lateral eye of Limulus was first described by Grenacher, and further and more accurately by Lankester and Bourne (5) and by Watase; that of Scorpio by Lan kester and Bourne, FIG.
--Diagrams of the meta-sternite st, who showed that with genital operculum op, and the first lamellithe statements of gerous pair of appendages ga, with uniting von Graber were sternal element st of Scorpio (left) and Limulus erroneous, and (right).
That the lateral eyes of Scorpio have a single cell-layered or " monostichous " ommatidium like that of Limulus.
(For details the reader is referred to Watase (11) and to Lankester and Bourne (5).) The structure of the central eyes of Scorpio and spiders and also of Limulus differs essentially from that of the lateral eyes in having two layers of cells (hence called diplostichous) beneath the lens, separated from one another by a membrane (figs.
It shows in Scorpio and Limulus a tendency to segregate into minor groups or " ommatidia."
The retinal cells of both the lateral and central eyes of Limulus and Scorpio produce cuticular structures on their sides; each such piece is a rhabdomere and a number (five or ten) uniting form a rhabdom (fig.
It is a very significant fact that the lateral and central eyes of Limulus and Scorpio not only agree each with each in regard to their monostichous and diplostichous structure, but also in the formation in both classes of eyes of rhabdomeres and rhabdoms in which the component pieces are five or a multiple of five (fig.
(From Lankester, loc. cit.) forming a star-like chitinous centre in section, each lateral eye of Scorpio has several rhabdoms of five or less rhabdomeres, indicating that the Limulus lateral eye-unit is more specialized than the detached lateral eyelet of Scorpio, so as to present a coincidence of one lens with one rhabdom.
Whilst Limulus agrees thus closely with Scorpio in regard to the VII?tg VII, The genital operculum.
VIII, The pectens of Scorpio and the first branchial plate of Limulus.