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.
D, Optical section of a branch of organs are present to the number of a single pair per somite, and are commonly present in the majority of the segments of the body, failing often among the Oligochaeta in a varying number of the anterior segments.
VII, Usually considered to be the tergum of the genital somite, but suggested by Pocock to be that of the other [According to the system of numbering explained in the text, if VII is the tergum of the praegenital somite (as is probable) it should be labelled Prg without any number, and the somites VIII to XIII should be lettered 1 to 6, indicating that they are the six normal somites of the mesosoma; whilst XV to XVIII should be replaced by the numbers 7 to 12 - an additional suppressed segment (making up the typical six) being reckoned to the metasomatic fusion.] (From Lankester, Q.
There is no doubt that these are parapodial or limb appendages, carrying numerous imbricated secondary processes, and therefore comparable in essential structure to the leaf-bearing plates of the second meso somatic somite of Limulus.
7, excepting that VII is here certainly the tergum of the first somite of the mesosoma - the genital somite - and is not a survival of the embryonic praegenital somite.
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.
17 and 18), or, if we please so to term it, a praegenital somite, hitherto unrecognized.
As in Limulus, so in Scorpio, this unexpected somite and its appendages disappear in the course of development.
In fact, more or less complete " excalation " of the somite takes place.
Owing to its position it is convenient to term the somite which is excalated in Limulus and Scorpio " the praegenital somite."
Hansen (10) has recognized that the " praegenital somite " persists in a rudimentary condition, forming a " waist " to the series of somites in the Pedipalpi and Araneae.
The present writer is of opinion that it will be found most convenient to treat this evanescent somite as something special, and not to attempt to reckon it to either the prosoma or the mesosoma.
When the praegenital somite or traces of it are present it should not be called " the seventh prosomatic " or the " first mesosomatic," but simply the " praegenital somite."
But it is necessary to remember, in the light of recent discoveries, that the sixth prosomatic pair of appendages is carried on the seventh somite of the whole series, there being two prosthomeres or somites in front of the mouth, the first carrying the eyes, the second the chelicerae; also that the first mesosomatic or genital somite is not the seventh or even the eighth of the whole series of somites which have been historically present, 1 See the article Arthropoda for the use of the term " prosthomere."
We shall, therefore, ignoring the ocular somite, speak of the first, second, third, fourth, fifth and sixth legbearing somites of the prosoma, and indicate the appendages by the Roman numerals, I, II, III, IV, V, VI, and whilst ignoring the praegenital somite we shall speak of the first, second, third, &c., somite of the mesosoma or opisthosoma (united mesosoma and metasoma) and indicate them by the Arabic numerals.
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.
VIIgo, The genital somite or first somite of the mesosoma with the genital operculum (a fused pair of limbs).
Y, The broad first somite of the metasoma.
Both belong to the category of " coelomoducts," namely, r- ' tubular or funnel-like portions of the coelom opening to the exterior in pairs in each somite (potentially,) and usually persisting in only a few somites as either "urocoels" (renal organs) or "gonocoels"(genital tubes).
In Peripatus they occur in every somite of the body.
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.
Which connect it with the IX, The second mesosomatic somite body-wall, both dorsally and or pectiniferous somite.
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.
PrGabp l, Rudiment of the appendage of the praegenital somite which disappears.
VIII, The first mesosomatic somite VIIPrG go VIII Km; ?
The praegenital somite, VII PrG, is still present, but has lost its rudimentary appendages; go, the genital operculum, left half; Km, the left pecten; abp 4 to abp 7, the rudimentary appendages of the lung-sacs.
The seventh, VII, is anterior to the genital operculum, op, and is the cavity of the praegenital somite which is more or less completely suppressed in subsequent development, possibly indicated by the area marked VII in fig.
Opening in pairs in each somite, right and left into the pericardial sinus are large veins, which bring the blood respectively from the gill-books and the lungbooks to that chamber, whence it passes by the ostia into the heart.
In both animals the wall of the pericardial sinus is connected by vertical muscular bands to the wall of the ventral venous sinus (its lateral expansions around the lung-books in Scorpio) in each somite through which the pericardium passes.
31) of the second (pectiniferous) mesosomatic somite; this is the most anterior pair of the series of six, none are present in the genital somite.
Md, Muscle from tergite of genital somite to entosternum (same as dpm in fig.
The genital apertures are placed in the first somite following the prosoma, excepting where a praegenital somite, usually suppressed, is retained.
The body and head have the two pleural regions of each somite flattened and expanded on either side of the true gut-holding body-axis.
This telson may enlarge, it may possibly even become internally and sternally developed as partially separate somites, and the tergum may remain without trace of somite formation, or, as appears to be the case in Limulus, the telson gives rise to a few well-marked somites (mesosoma and two others) and then enlarges without further trace of segmentation, whilst the chitinous integument which develops in increasing thickness on the terga as growth advances welds together the unsegmented telson and the somites in front of it, which were previ ously marked by separate tergal thickenings.
On the other hand, an unusually large tergal plate, whether terminal or in the series, is not always due to fusion of the dorsal plates of once-separate somites, but is of ten a case of growth and enlargement of a single somite without formation of any trace of a new somite.
A, Restored thoracic limbs in transverse section of the animal; b, section across a posterior somite; c, section across one of the sub-terminal somites.
A somite placed between the prosoma and mesosoma - the prae-genital somite - appears to have belonged originally to the prosomatic series (which with its ocular prosthomere and palpi FIG.
(After Beecher, from Zittel.) retained as a rudimentary, separate, detached somite in front of the mesosoma, or disappears altogether (excalation).
The seventh, and sometimes the eighth, leg-bearing somite is present and has its leg-like appendages fully developed.
- These start from highly developed and specialized aquatic branchiferous forms, exhibiting a prosoma with six pediform pairs of appendages, an intermediate prae-genital somite, a mesosoma of six somites bearing lamelliform pairs of appendages, and a metasoma of six somites devoid of appendages, and the last provided with a post-anal spine.
They open on the first somite of the mesosoma.
The prae-genital somite partially or wholly obliterated in the adult.
The dotted line on somite I indicates the position of the genital operculum which was probably provided with branchial lamellae.] well-developed somites is present and the posterior ones form a long tail-like region of the body.
The prae-genital somite, after appearing in the embryo, either is obliterated (Scorpio, Galeodes, Opilio and others) or is retained as a reduced narrow region of the body, the " waist," between prosoma and mesosoma.
2nd) somite (Opiliones) of the opisthosoma, there being rarely an additional stigma on the 4th (some Solifugae).
The appendages of the 2nd somite of the opisthosoma absent, rarely minute and budlike (some Amblypygi), never pectiniform.
A prae-genital somite is often present either in a reduced condition forming a waist (Pedipalpi, Araneae, Palpigradi) or as a full-sized tergal plate (Pseudoscorpiones); in some it is entirely atrophied (Solifugae, Holosomata, and Rhynchostomi).
A narrow prae-genital somite is present between opisthosoma and prosoma (figs.
Intromittent organ of male beneath the genital operculum (=sternum of the 1st somite of opisthosoma).
- Thelyphonus sp. Ventral view of the anterior portion of the body to show the three prosomatic sternal plates a, b, c, and the rudimentary sternal element of the praegenital somite; opisth 1, first somite of the opisthosoma.
- The possibility of another interpretation of the anterior somites of the mesosoma and the prae-genital somite must be borne in mind.
Excalation of one or of two anterior mesosomatic somites, besides the prae-genital somite, would then have to be supposed to have occurred also.
1 1, The eleventh somite of the opisthosoma.
Opisth, First somite of the opisthosoma.
11 opisth, Eleventh somite of the opisthosoma.
Pregen, Position of the praegenital somite (not seen).
Thelyphonusassamensis d .Ventral surface of theanteriorregionof the opisthosoma, the first somite being pushed upwards and forwards so as to expose the subjacent structures.
Opistho I, First somite of the opisthosoma; opistho 2, second do.; g, genital aperture; 1, edges of the lamellae of the lung-books; m, stigmata of tergo-sternal muscles.
Opisthosoma when segmented showing the same number of somites as in the Pedipalpi; usually unsegmented, the prae-genital somite constricted to form the waist; the appendages of its 3rd and 4th somites retained as spinning mammillae.
1, The sternum of the first opisthosomatic or genital somite covering the V "' genital aperture and the first pair of lung sacs.
In front of it the narrow waist is formed by the soft sternal area of the praegenital somite; 2, the sternite of the 2 second opisthosomatic somite covering the posterior pair of lung-sacs; and 4, the spinning appendages (limbs) of the opisthosoma; a, inner, b, outer ramus of the appendage; I I, sternite of the eleventh --