ECHINODERMATA. 457 



(2) The gastrula opening forms the permanent anus, and the 

 mouth is formed by a fresh invagination. 



(3) The mesoblast arises entirely from the invaginated cells, 

 but in two ways : 



(a) As scattered amoeboid cells, which give origin to the mus- 

 cles and connective tissue (including the cutis) of the body wall and 

 alimentary tract. 



(b) As a portion separated off from the archenteron, which 

 gives rise both to the epithelioid lining of the body cavity, and of 

 the water-vascular system. 



(4) The oesophagus is derived from an invagination of the 

 epiblast, and the remainder of the alimentary canal from the archen- 

 teron. 



(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 Metsclmikoff (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 (tig. 260, i), and a madreporic 

 canal (h) 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 pos- 

 terior 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 Holu- 

 thuriaiis, becomes the oesophagus of the adult. The posterior vesicles 

 give rise to the lining of the body cavity, but are stated by Metschnikotf 



