ECHINOCYAMUS PUSILLUS. 41 



length, run parallel and become connected by transverse beams. With 

 the increase of the larva itself and its arms, the latticed rods extend 

 in length and become spinous. 



Thus, each of the two centres of calcification has given rise to 

 a very complicated spicule on each side of the Pluteus, which is built up 

 of two posterior rods supporting the under half of the body, one un- 

 paired one running towards the ventral middle-line, one passing for- 

 wards to support the upper part of the larva, and one latticed rod ex- 

 tending into the larval arms. The two last rods on each side hold the 

 concave part of the bell in a state of expansion. 



The Pluteus in this first stage reaches its maximum size at 

 about six days after the fecundation, PL V, fig. 82 — 85, and after this 

 a short period of repose in the development seems to take place. The 

 following measurements of a Pluteus, about six or seven days old, may 

 be noticed : 



Length from the posterior end of the body to the points of the posterior arms 



= 0,4 mm. 

 Length of the body itself including the anterior arms = 0,26 mm. 

 Length from the posterior end of the body to the base of the posterior arms 



= 0,16 mm. 



With regard to the external shape of the Pluteus in this stage, 

 there are some variations, the under half of the body being either 

 conical or cylindrical with broad obtuse end, or even spherical. In con- 

 nection herewith, certain deviations take place in the reciprocal position 

 and communication of the supporting rods. Sometimes I also found that 

 the latticed rods rise with a simple basal staff, Pi!. F, fig. 85 a. 



Meanwhile, the invaginated entodermic sac has undergone con- 

 siderable changes. The blastopore becomes removed far from the poste- 

 rior end of the body, and the archenteron itself, at first a straight cy- 

 linder, curves and grows towards that point of the ventral surface, 

 where its blind end subsequently meets the ectodermic invagination in 

 order to form a communication with the exterior, PL III, fig. 41. 

 Even before the junction of the oral and anal involutions as well as 

 before the formation of the vaso-peritoneal vesicles, the archenteron 

 shows three different portions marked by slight constrictions, PL III., 

 fig. 42 — 45, which eventually give rise to the intestine, the stomach 

 and the vaso-peritoneal vesicles, and pi-obably also partake in the for- 

 mation of the oesophagus. 



Nova Acta Reg. Soc. Sc. Ups. Ser. III. 6 



