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.
1 to 6) the entosternites of Limulus, Scorpio and Mygale.
- The first three pairs of mesosomatic appendages of Scorpio and Limulus compared.
- The remaining three pairs of mesosomatic appendages of Scorpio and Limulus.
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).
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.
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.
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 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.
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.
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.
- 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.
(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.
IX, The first pair of lung-books of Scorpio and the second branchial plate of Limulus.
Microscopically their structure is the same in essentials as that of the coxal glands of Scorpio (13).
Similar observations were made by Laurie (17) in Lankester's laboratory (1890) with regard to the early condition of the coxal gland of Scorpio, and by Bertkau (41) as to that of the spider Atypus.
Thus an organ newly discovered in Scorpio was found to have its counterpart in Limulus.
The coxal glands do not establish any special connexion between Limulus and Scorpio, since thay also occur in the same somite in the lower Crustacea, but it is to be noted that the coxal glands of Limulus are in minute structure and probably in function more like those of Arachnids than those of Crustacea.
In Limulus small entosternites are found in each somite of the appendage-bearing mesosoma, and we find in Scorpio, in the only somite of the mesosoma which has a welldeveloped pair of appendages, that of the pectens, a small entosternite with ten pairs of muscles inserted into it.
In Scorpio the completion of the horizontal plate by oblique flaps, so as to form an actual diaphragm shutting off the cavity of the prosoma from the rest of the body, possibly gives to the organs contained in the anterior chamber a physiological advantage in respect of the supply of arterial blood and its separation from the venous blood of the mesosoma.
- The blood fluids of Limulus and Scorpio are very similar.
This body occurs also in the blood of Crustacea and of Molluscs, but its abundance in both Limulus and Scorpio is very marked, and gives to the freshly-shed blood a strong indigo-blue tint.
The great dorsal contractile vessel or " heart " of Limulus is closely similar to that of Scorpio; its ostia or incurrent orifices are FIG.
(After Lankester.) placed in the same somites as those of Scorpio, but there is one additional posterior pair.
V., 1892.) heart differs in Limulus from the arrangement obtaining in Scorpio, in that a pair of lateral commissural arteries exist in Limulus (as described by Alphonse Milne-Edwards (6)) leading to a suppression of the more primitive direct connexion of the four pairs of posterior II.