6 Echinoderma. 



and interambulacral regions are not radially constructed ; and by the Pelmatozoa 

 from that of a Cystid with few plates. But no homology is traceable between indi- 

 vidual plates of the abactinal system in the 2 groups ; though the Miillerian doc- 

 trine that the terminal plates of Asterids are homologous with the oculars of 

 Echinids will still. hold good. 



II. Felmatozoa 



(including Crinoidea, Cystidea, and Blastoidea) . 



See also Barrels, supra, p 2, Bury, supra, p 3, Neumayr, supra, p 5, the 

 Sarasins ( 3 ), supra, p 4, and Semon, supra, p 3. 



According to Barrois the segmentation of the ovum in Antedon rosacea gives 

 rise to a mass of 32 blastomeres, 8 of which, larger than the rest and situated at 

 the vegetative pole, give rise to the endoderm, while the remaining 24 produce 

 the ectoderm. The segmentation cavity is connected with the exterior by an 

 orifice at each pole, which is obliterated by further cleavage ; while the distinction 

 between the cells at the formative and vegetative poles also disappears and a 

 normal archiblastula is developed, passing into a normal gastrula with the en- 

 doderm slightly bent to one side. It consists of narrow cylindrical cells which are 

 at first in a single layer, but subsequently become more irregular and 2 or 3 deep, 

 the outer layer being detached as mesenchym cells which accumulate round the 

 blind end of the archenteron. The blastopore closes towards the end of the second 

 day and the endodermic vesicle, which is detached from the ectoderm, becomes 

 divided by a median constriction into 2 nearly equal parts. These are situated in 

 the superior half of the embryo, which gives rise to the calyx ; while the inferior 

 half, filled by mesenchym cells, becomes the stalk and represents the pre-oral 

 lobe of other Echinoderm larvae. The superior endodermic vesicle now increases 

 in breadth, becoming constricted to form 2 peritoneal sacs ; while from its junction 

 with the inferior vesicle there grow out 2 prolongations which enclose the latter 

 like a crescent, a smaller and transitory one on the ventral side, and a larger one 

 on the dorsal side, the future gut, which terminates blindly. The endoderm is 

 now thinner than the ectoderm, a thickening of which on the ventral side is the 

 first trace of the future vestibular depression. Traces of the ciliated bands 

 appear, wide at first but afterwards more narrow. The ciliated cells diverge in- 

 wards with well marked nuclei, while the remaining ectoderm cells diverge out- 

 wards and their nuclei degenerate into granules. There are 4 bands and a ter- 

 minal cap, which consists of 2 parts, separated from one another by colourless 

 ectoderm, viz. a central one bearing a terminal tuft and an incomplete fifth ring 

 of non-ciliated cells, arranged in a less distinct fan-like manner than in the com- 

 plete bands, the cells of which have many nuclei and striated outer borders. 

 The inferior endodermic vesicle now divides into the hydrocoel on the left, and 

 a smaller cavity on the right which gives rise to the water-tube. This is con- 

 nected with the hydrocoel, which eventually places itself in the middle line at the 

 end of the gut. Right and left of the latter are the 2 peritoneal sacs and the 

 whole visceral mass occupies the median longitudinal axis of the embryo. But it 

 eventually shifts this position for one parallel to the shorter axis, turning on itself 

 at the same time through a half circle so that the horseshoe-like hydrocoel gains 

 a ventral position, while the ventral and inferior part of the embryo becomes 

 marked by a great depression or vestibular fossa. This is deepest at the in- 

 ferior end, and directly beneath it are the water-tube and the right peritoneal sac. 

 The latter is separated from the left sac by 2 mesenteries, formerly anterior and 

 posterior, but now right and left. The hydrocoel now becomes five-lobed and its 



