ECHINODERMATA. 549 



(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, z), and a madreporic canal (k) with a dorsal 

 pore opening to the exterior. The rosette appears not to grow round the 

 oesophagus, as in the cases hitherto described. But the latter is stated to 

 disappear, and a new oesophagus 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 larvae 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 oesophagus, 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 oesophagus, which, as in Holothu- 

 rians, becomes the oesophagus 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 develop- 

 ment of the -vase-peritoneal bodies takes place as in the free-swimming 

 larvae. 



Echinoidea. In the Echinoidea (Agassiz, No. 542, Selenka, No. 564) 

 there is a regular segmentation and the normal invagination (fig. 250 A). 

 The amoeboid mesoblast cells arise as two laterally placed masses, and give 

 rise to the usual parts. The archenteron grows forward and bends towards 



