FORMAT工ON OF STOMODAEUM AND NERVOUS SYSTEM 工N THE HORSESHOE CRAB，
TACHYPLEUS TR工DENTATUS (CHEL工CERATA: MEROSTO
蕊A
TA: X工PHOSUJそIDA)

Tatsuya Masuda and Tomio Itow
Department of Biology，Faculty of Education，Shizuoka University，
Shizuoka，422 Japan

Inhibitors of DNA synth日 sis induce the monsters with separated embryonicar日品目 which are separat自 d into anterior and posterior pieces.

Also in these. In th日 normal horseshoe crab，mon日 ters，the position of brain is abnormal
Tachypleus tridentatus the alimentary canal passes through the central nervou日
日yst日 m.

Howev色 r，this cros日ing of alim巴ntary canal and central nervous syst日 m

is lost in some monsters (Itow， 1982，ヲ 85).
We hoped to solve the mechanism of form昌 tion of stomoda壱 um and n告 rvous
system by m告 ans of the an昌 lysing the mons七色 rs. Bu七 皆 e did no七 clearly know
th自 process of formation of stomod丑 eum and nervous system in the norm呂 lembryos，neither.

Ther日 for日

we 巴 xamined th日 proc日 ss of the stomoda日 um

formation in normal embryos at first.

In this report， we describe th日

formation of alimentary canal and central nervous sy日t日m of normal embryos andlarvae.
We found new following facts，concerning the formation of alimentary canaland central nervous system of T. 七ridentatus.
1) The stomod丑 eum is a tubu工ar structure and app告 ars in front of thefirst cephalothoracic appendages at stage 14 (th告 stage of appearance ofcephalothoracic appendages).
2) The mouth migrat白日 posteriorlyat stage 18 (the stag巴 after th巴 1stembryonic moulting).

工t becomes situated in th日 position between the 3rd

C日 phalothoracic appendage日

and 4th ones，after th日 3rd embryonic moulting(stage 20).
3) The n邑 rvous system seems to be formed aft思 r stage 1
1 and 12 (the2526stage of active morphogenetic movement).

The ce11ωmasses of the nervous

system are c1ear1y observed after stage 16 (the stage of deve10pment of
cepha1othoracic 昌 ppendages).

After stage 19 (the stage after the 2nd

embryonic mou1ting)，the ce11 masses of nervous syst日 m are easi1y stained by伊eosln.
4) Adu1t horseshoe crabs have 7 pairs of 昌 bdomina1 appendages and 10
pairs of abdomina1 segments.

The 1st instar 1arva日 have on1y 4 pairs ofabdomina1 appendages，but have 16 pairs of gang1ia (6 cepha1othoracic and 10
abdomina1 ones). The number is the same as that of adu1ts.

That is，the 1stinst昌 r 1arvae a1r告 ady have the same number of segm告 nts 畠 S 丑du1ts.
5) There was the thought tha七 extra segments existed at the position
between the prosome and opistho日ome，and w日 re thought to have no appendages.
However，our detailed obs巴rvation showed that there never be such a segment.
6) There is the commissure of ganglion of the 6th cephalothoracic
segmen七s in the 1st and 2nd instar larva由Shoji (192ヲ) said the 品 dults do

not have the commissure.
7) It has b巴 en known that all cephalothoracic appendages and the 1stabdominal appendag告 s Ichil呂 rias) belong to the prosome.
following fact.

We also found theThe ganglion of the 2nd 品 bdominal segment exists in th臼

prosome of the 1st and 2nd instar larvae.

Reference器工tow，T. (1ヲ82) Deve1op. Growth Differ.，24:295‑303.

，
官 T. (1ラ82) Zool. ド1昌g.，ヲ1:230‑238.

工七0工 n press).
Itow，T. (1985) Acta. Embryol. Morphol. Exp.，(工Shoji， K. (1929)Th日 horseshoe crab.Compilation of Animal Anatomy 11，(Y，Okada)，109ω160. Kyoritsu，Tokyo.

Proc.Arthropoa.Embnl
口1.Soc.Jpn.，
1985