- Entosternum, entosternite or plastron of Limulus Polyphemus, Latr.
- Stra ussDiirckheim (1) was the first to insist on the affinity between Limulus and the Arachnids, indicated by the presence of a free suspended entosternum or plastron or entosternite in both.
A minute entosternite having the above-described structure is found in the Crustacean Apus between the bases of the mandibles, and also in the Decapoda in a similar position, but in no Crustacean does it attain to any size or importance.
On the other hand, the entosternite of the Arachnida is a very large and important feature FIG.
In each somite of the mesosoma is a small, free entosternite having a similar position, but below or ventral to the nerve cords, and having a smaller number of muscles attached to it.
The entosternite was probably in origin part of the fibrous connective tissue lying close to the integument of the sternal surface - giving attachment to muscles corresponding more or less to those at present attached to it.
Subsequently to the floating off of the entosternite the approximation of the nerve cords took place in the prosoma, and thus they were able to take up a position below the entosternite.
3 and 4) the entosternite has tough membrane - like outgrowths or genital somite.
Only the central and horizontal parts of this structure correspond precisely to the entosternite of Limulus: the right and left anterior processes(marked ap in figs.
The scorpion's entosternite gives rise to outgrowths, besides the great posterior flaps, pf, which form the diaphragm, unrepresented in Limulus.
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.
The supra-pectinal entosternite lies ventral to the nerve cords.
5 and 6) the form of the entosternite is more like that of Limulus than is that of Scorpio.
Muscular fibres connected with the suctorial pharynx are in Limulus inserted into the entosternite, and the activity of the two organs may be correlated.
An internal skeletal plate, the so-called " entosternite " of fibrocartilaginous tissue, to which many muscles are attached, is placed between the nerve-cords and the alimentary tract in the prosoma of the larger forms (Limulus, Scorpio, Mygale).
Entosternite: - Schimkewitsch, " Bau and Entwick.
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