(5) The embryonic systems of organs pass directly into those of the adult.
The development of Synapta diverges, as might be expected, to a very small extent from that of Holothuria.
Asteroidea. In Asterias the early stages of development conform to our type. There arise, however, two bilaterally symmetrical vaso-peritoneal diverticula from the archenteron. These diverticula give rise both to the lining of the body cavity and water-vascular system. With reference to the exact changes they undergo there is, however, some difference of opinion. Agassiz (543) maintains that both vesicles are concerned in the formation of the water-vascular system, while Metschnikoff (560) holds that the water-vascular system is entirely derived from the anterior part of the larger left vesicle, while the right and remainder of the left vesicle form the body cavity. Metschnikoff’s statements appear to be the most probable. The anterior part of the left vesicle, after separating from the posterior, grows into a five-lobed rosette ([fig. 260], i), and a madreporic canal (h) with a dorsal pore opening to the exterior. The rosette appears not to grow round the œsophagus, as in the cases hitherto described. But the latter is stated to disappear, and a new œsophagus to be formed, which pierces the rosette, and places the old mouth in communication with the stomach. Except where the anus is absent in the adult, the larval anus probably persists.
Ophiuroidea. The early development of the Ophiuroidea is not so fully known as that of other types. Most species have a free-swimming larva, but some (Amphiura) are viviparous.
The early stages of the free-swimming larvæ have not been described, but I have myself observed in the case of Ophiothrix fragilis that the segmentation is uniform, and is followed by the normal invagination. The opening of this no doubt remains as the larval anus, and there are probably two outgrowths from this to form the vaso-peritoneal vesicles. Each of these divides into two parts, an anterior lying close to the œsophagus, and a posterior close to the stomach. The anterior on the right side aborts; that on the left side becomes the water-vascular vesicle, early opens to the exterior, and eventually grows round the œsophagus, which, as in Holothurians, becomes the œsophagus of the adult. The posterior vesicles give rise to the lining of the body cavity, but are stated by Metschnikoff to be at first solid, and only subsequently to acquire a cavity—the permanent body cavity. The anus naturally disappears, since it is absent in the adult. In the viviparous type the first stages are imperfectly known, but it appears that the blastopore vanishes before the appearance of the mouth. The development of the vaso-peritoneal bodies takes place as in the free-swimming larvæ.
Echinoidea. In the Echinoidea (Agassiz, No. [542], Selenka, No. [564]) there is a regular segmentation and the normal invagination ([fig. 250] A). The amœboid mesoblast cells arise as two laterally placed masses, and give rise to the usual parts. The archenteron grows forward and bends towards the ventral side ([fig. 250] B). It becomes ([fig. 250] C) divided into three chambers, of which the two hindermost (d and c) form the stomach and intestine; while the anterior forms the œsophagus, and gives rise to the vaso-peritoneal vesicles. These latter appear as a pair of outgrowths ([fig. 251]), but become constricted off as a single two-horned vesicle, which subsequently divides into two. The left of these is eventually divided, as in Asteroids, into a peritoneal and water-vascular sack, while the right forms the right peritoneal sack. An oral invagination on the flattened ventral side meets the mesenteron after its separation from the vaso-peritoneal vesicle. The larval anus persists, as also does the larval mouth, but owing to the manner in which the water-vascular rosette is established the larval œsophagus appears to be absorbed, and to be replaced by a fresh œsophagus.
Fig. 250. Three side views of early stages in the development of Strongylocentrus. (From Agassiz.)
a. anus (blastopore); d. stomach; o. œsophagus; c. rectum; w. vaso-peritoneal vesicle; v. ciliated ridge; r. calcareous rod.