ECHINODERMA GENERAL DESCRIPTION 17 



growing tube feet and spines causes it to break through the outer 

 wall (Fig. XV. 4). While this takes place the spicular skeleton of 

 the Pluteus is absorbed, and the body of the Pluteus shrinks up to 

 a sac on the aboral side of the young Echinoid (Fig. XV. 5). The 

 hydropore from the first opens on the dorsal surface, which becomes 

 the aboral side. The right posterior coelom is also under here, 

 as in Stelleroids. The larval stomach becomes that of the adult, 

 but a fresh mouth is formed in the centre of the hydrocircus, 

 while the anus is a fresh formation at the aboral pole. 



It is easy to understand that, with this amniotic development 

 in the body of the larva, most of the traces of the Pentactcea 

 stage have disappeared. There is, however, evidence of a preoral 

 lobe, while the coil of the intestine and the radiate structure of 

 the hydrocoel, nerves, and gonads, bear witness to antecedent 

 phylogenetic changes. On those changes light is thrown by 

 palaeontology, which teaches us that the primitive Echinoid had a 

 spheroidal body, with muscular, flexible walls, in which irregular 

 plates were developed ; the mouth was at the centre of the lower 

 surface ; the anus on the upper surface, and near it the madre- 

 porite (the successor of the hydropore). Combining with the 

 svidence from fossils that from comparative anatomy, we infer 

 that the gut had a simple dextral coil ; that the oesophagus was 

 surrounded by three rings water-vascular, blood-vascular, and 

 nervous ; and that from each ring five branches passed up the 

 inside of the body wall to the aboral pole ; that branches from the 

 radial water-vessels passed, between the plates in the body wall, 

 to the exterior, and became suckers assisting locomotion, the 

 complete structures being ambulacra ; that gonads were five, 

 unpaired, and interradially disposed in the body cavity. Such a 

 form had lost the stem of the Pentactcea, and had never possessed 

 an apical system of plates. It had, however, already developed 

 food-grooves, with nerves and ambulacral vessels, while there must 

 have been some radiate arrangement of the gonads. The sinking of 

 the nerves and closure of the food-grooves forming epineural canals 

 (Fig. XIII. 3) probably took place as we suppose it to have done 

 in Ophiuroids. Among Pelmatozoa, the Edrioasteroidea (p. 205) 

 present a structure removed from that of the primitive Echinoid 

 in little but the upward position of the mouth and (probably) the 

 madreporite, and the functional food-grooves ; the notable point about 

 the latter is the presence of openings between the flooring-plates, 

 apparently for the passage of processes from the radial water- vessels. 



The peculiarities in the structure and development of the 

 Holothurians may perhaps be ascribed to their having in many 

 respects regressed from the Pelmatozoic towards the Dipleurula 

 type (Fig. XVI.). Thus the mouth has again come to lie at one 

 end of the body, while the anus is at the other. With the mouth 



