KTJNGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 49. N:0 4. 9 



with new formation of ealcareous substances, and that these processes are facilitated 

 by the dissolved salts being directly transferred through the pseudopodial intercom- 

 munication from one place to another.» 1 



II. The second subject of investigation which is to be treated here relätes to 

 the shape and aspect of the hydrocoele in its earlier stage of development, when it 

 has separated from the left posterior coelom and is going to meet the ectodermic 

 invagination in order to establish together with it and the coelom the main portion 

 of the future sea-nrchin. I have reason for subjecting this question to a fresh dis- 

 cussion, inasmuch as Mac Bride 2 and myself 3 seem to entertain different views of 

 the matter. Thus in 1902 Mac Bride wrote (p. 272): »At 19—20 days the ecto- 

 dermic invagination on the left side of the larva has become flask-shaped, and its 

 floor is novv in close contact with the hydrocoele. The latter has now taken on the 

 form of a flattened pentagon (fig. 6). It never has the form of an open hoop, so cha- 

 racteristic to the corresponding organ in Asteroidea, Ophiuroidea and Holothurioidea. 

 And fnrther on he says (p. 275): »The first trace of the adult mouth and oesophagus 

 appears as an invagination of the oral disk, indenting the centre of the hydrocoele. At 

 a låter period it completely perforates the hydrocoele, thus converting it from a flattened 

 plate into a ring». 4 



From the above quotation on would naturally gather that the Echinoids form 

 an exceptional case. This is certainly not the case with Parechinus miliaris and, in 

 my conviction, never in other forms of Echinoids either. 



In 1902 5 I undertook an investigation of the matter in qnestion, and I explain- 

 ed my views in the following manner: »The floor [of the flask-shaped ectodermic 

 invagination = the primary amniotic cavity], also called oral disc by Mac Bride, 

 which is destined to be transformed into oesophagus, nervous ring etc, is first round- 

 ed in shape but subsequently acquires a pentagonal form. I prefer to call it solely 

 »ectodermic disc-». The hydrocoele, being almost in contact with the pentagonal floor 

 of the amniotic cavity, has become transformed into a hollow ring open posteriorly 

 and provided with five hollow processes which constitute the first traces of »radicd 

 canals» of the future Echinus. Låter on the ring becomes closed posteriorly.» 



Thus I stated that the hydrocoele first forms a hoop open posteriorly, just as 

 is the case in the Holothurids etc. and that the hoop subsequently becomes converted 

 into a complete ring. 



1 Théel, loc. cit. p. 57. 189G. 



- The development of Echinus esculentus. — Proceed. Roy. Soc. London. Vol. G9, 1902, pp. 268—276. 



The development of Echinus esculentus together with some points in the Development of E. miliaris and E. 



acutus. — Phil. Träns. Roy. Soc. London. Ser. B. Vol. 195. London 1903, pp. 285—327, pl. 7—16. 



3 Preliminary account of the development of Echinus miliaris L. with tlnee double-plates. — Bihang, 

 K. Svenska Vet. Akad. Handlingar, Stockholm 1902. Bd. 28. Afd. 4. N:o 7, pp. 1 — 11. 



4 Ralics by the author. 



5 loc. cit. 1902. p. 5. 



K. Sv. Vet. Akad. Haudl. Band. 49. No 4. 2 



