524 



COMPARATIVE ANATOMY 



CHAP. 



and the amnion, together with part of the larval integument, are lost when the 

 larva changes into the young Echinoid. 



The larval arms disajjpear, and their spicules are for the most part absorhed. 

 As a rule, one or other of the arms of the Pluteus still adheres to tlie quite young 

 Echinoid (Fig. 422). 



The intestine, at least the whole stomach, the spreading enteroca'l, and the 

 growing hydroca-1 are taken over into the young Echinoid ; the latter, however, 



has, at first, neither month nor 

 anus. In Echinoids, therefore, 

 the larval mouth and anus do not 

 pass direct into the corresponding 

 organs of the adult. 



Formation of the mouth and 

 the definitive oesophagus. — Ac- 

 cording to one account, the oeso- 

 phagus only grows out from the 

 intestine after the horse - shoe- 

 shaped hydroccel has closed ; it 

 then passes through the water 

 vascular ring and opens outward 

 at the centre of the Echinoid disc 

 through the definitive mouth. 



The pedicellarise arise A'ery 

 early. Thej' are even occasionally 

 seen on the dorsal side of an older 

 Pluteus larva. 



The water pore becomes the 

 madreporite, and the unpaired 

 spicule which, in the older Pluicns, 

 arose in it.s immediate neighbour- 

 hood, changing into a lattice-like 

 plate, becomes the madreporitic 

 basal. Four other plates, which 

 arise over the right enterocoel 

 oC the larva, become the other basals. In their centre the dorso-central is soon 

 distinguishable. On the oral side, in the peripheral part of the original Echinoid 

 disc, where the primary tube-feet developed, the first ambulacral and interambu- 

 lacral plates appear, with the rudiments of the spines and the sphseridia, both 

 of which form independently over the plates (Fig. 423). In the future oral area, 

 which is surrounded by a circle of ambulacral and interambulacral plates, thirty 

 small calcareous centres form, three in each radius and three iu each interradius ; 

 these are the rudiments of the plates of the masticatory apparatus. The middle 

 calcareous plates of the interradii become the teeth. 



Little or nothing is known of the ultimate fates of the other twenty-five pieces, or 

 of the enterocic.l, of the hydroccel {e.g. the order of appearance of the tube-feet), or as 

 to the appearance of the nervous system, the origin of the radial plates, etc. 



Fiii. 423.— Eohinocyamus puslUus, young Ecliinoid, 

 about forty-tlve days old, from tlip oral side (after Th^el). 

 ant, Anterior unpaired ambulacrum; poi;^ posterior nii- 

 liaired interambulacrum ; 1, tentacle ; 2, spines ; 3, spliasr- 

 iilia in their niches ; 4, pieces of the njasticatory apparatus ; 

 T), teeth ; 6, oral integument, the mouth is not yet formed ; 

 7, radial skeletal plates ; s, interradial skeletal plates. 



D. Ontogeny of the Asteroidea. 



Segmentation is total, and leads to the formation of a coeloblastula, through the 

 invagination of which a ccelogastrula arises. The formation of the mesenchyme 



takes place in the manner already described for the Holofliuriublca and the Echin- 



