ECHINOCYAMUS PUSILLUS. 25 



intestine increases not only by means of the process of invagination, 

 but also independently. 



The separation of wandering cells from the walls of the archenteron 

 to form the mesenchyme is going on during the whole process of ga- 

 strulation, but they appear seldom to arise from the blind end itself of 

 the archenteron, PL III, fig. 52 — 51 . The number of such cells increases 

 considerably during the gastrulation and some of them constitute fine 

 filaments connecting the ectoderm with the archenteron. 



However, the Gastrula also has changed in its external appearance, 

 which has in a certain degree been due to the development of the 

 two calcareous spicules and their increase, which will be treated more 

 in detail further on. From each of the two centres of calcification a 

 calcareous rod grows out towards the ectoderm, compelling this to pro- 

 trude, PI. Ill, fig. 38, and thus giving rise to the first indication of the 

 two posterior ventral arms of the future Pluteus. At the same time as the 

 Gastrula has obtained these two obtuse prominences, one on each side 

 and slightly in front of the blastopore, it undergoes other changes. 

 The dorsal surface becomes slightly convex, the ventral surface on the 

 contrary concave, and the blastopore, which has had a terminal position, 

 is moved slightly forwards on the latter surface, Pi. Ill, fig. 39 — 41. 



If we compare the Gastrula of Echinoeyamus with Gastrulee of 

 other Clypeastroids viz. those of Echinarachnius and Mellita, it seems 

 to be a fact that they differ in some more or less important points, that 

 of the Echinoeyamus being almost cylindrical, those of the other two 

 forms presenting apparently a spherical shape. Moreover, according to 

 Fewkes ') Echinarachnius seems quite to lack a differentiated cell-area 

 at the animal pole, while according to Nachtrieb ^) Mellita is provided 

 with a well developed one, just as is the case with the Gastrula of 

 Echinoeyamus. 



With regard to the shape of the Gastrula in other Echinoids, dif- 

 ferences of more or less importance may be noted. Thus, Selenka ') 

 states for instance, that in Echinus miliaris, Toxopneustes brevispi- 

 nosus and Strongylocentrotus lividus it is almost spherical, while in 

 Arbacia pustulosa and Echinocardium cordatum it is elongate and conical. 



1) Preliminary Observations ou the Development of Ophiopholis and Echi 

 uarachnius. 1886. p. 127—130. 



2) Preliminary Notes on the Echinoderms of Beaufort. 1885. p. 67. 



3) Keimblätter und Organanlage der Echiniden. 1879. p. 48. 



Nova Acta Reg. Soo. Sc. Ups. Ser. III. 4 



