- Entosternum, entosternite or plastron of Limulus Polyphemus, Latr.
Xxiv., 1884.) Limulus an Arachnid.
- 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.
N.S.), and in a series of subsequent memoirs, in which the structure of the entosternum, of the coxal glands, of the eyes, of the veno-pericardiac muscles, of the respiratory lamellae, and of other parts, was for the first time described, and in which the new facts discovered were shown uniformly to support the hypothesis that Limulus is an Arachnid.
- Ventral surface of the entosternum of Limulus Polyphemus, Latr.
47), as do Limulus and Scorpio.
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.
It is not desirable to occupy the limited space of this article by a full description of the limbs and segments of Limulus and Scorpio.
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).
14 and 15), each carrying a pair of plate-like appendages in both Limulus and Scorpio.
The tergites of this region and those of the following region, the metasoma, are fused to form a second or posterior carapace in Limulus, whilst remaining free in Scorpio.
This is represented in Limulus by the first gillbearing appendage.
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.
- Diagram of the dorsal surface of Limulus polyphemus.
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.
Aquatic respiration somewhat as in Limulus, though not necessarily repeating the exact E ??.
In Limulus the metasoma is practically suppressed.
In the embryo Limulus (fig.
42) the six somites of the mesosoma are not fused to form a carapace at an early stage, and they are followed by three separately marked metasomatic somites; the other three somites of the metasoma have disappeared in Limulus, but are represented (From Lankester, loc. cit.) by the unsegmented prae-anal region.
It is probable that we have in the metasoma of Limulus a case of the disappearance of once clearly demarcated somites.
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.
Of Limulus and Scorpio.
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.
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."
- Ventral view of the posterior carapace or meso-metasomatic (opisthosomatic) fusion of Limulus polyphemus.
(After Lankester, loc. cit.) the last pair of legs in both Scorpio and Limulus, viz.
10) and the chilaria of Limulus (see figs.
This has not been demonstrated by an actual following out of the development, but the position of these pieces and the fact that they are (in Limulus) supplied by an independent segmental nerve, favours the view that they may comprise the sternal area of the vanished praegenital somite.
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 position of the chelicerae of Limulus and of the ganglionic nerve-masses from which they receive their nerve-supply, is closely similar to that of the same structures in Scorpio.
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.
- Third leg of Limulus Polyphemus, showing the division of the fourth segment of the leg by a groove S into two, thus giving seven segments to the leg as in scorpion.
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 Crustacea have, in fact, three prosthomeres in the head and the Arachnida only two, and Limulus agrees with the Arachnida in this respect and differs from the Crustacea.
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.