524 



COMPABATIVE ANATOMY 



C-- 



and tlie anniion, together with part of tlie larval integument, are lost when tlie 

 larva changes into the young Eehinoid. 



The larval arms disappear, and their spicules are for the most jiart absorbed. 

 As a rule, one or other of the arms of the Pluteus still adheres to the (^uite voung 

 Eehinoid (Fig. 422). 



The intestine, at least the whole stomach, the spreading enteroccel, and the 

 growing hydrocad are taken over into the young Eehinoid ; the latter, however, 



has, at first, neither mouth nor 

 '^'*''- , anus. In Echinoids, therefore, 



the larval mouth and anus do not 

 l)ass direct into the corresponding 

 organs of the adult. 



Formation of the mouth and 

 the definitive oesophagus. — Ac- 

 cDi'ding to one aceoiint, the oeso- 

 ]iliagus only grows out from the 

 intestine after the horse - shoe- 

 shaped hydrocoel has closed ; it 

 then passes through the water 

 vascular ring and opens outward 

 at the centre of the Eehinoid disc 

 through the definitive mouth. 



The pedicellarise arise \'ery 

 early. They are even occasionally 

 seen on the dorsal side of an older 

 PJutciis larva. 



The water pore becomes the 

 428.-Echinocyamus pusillus, young Eehinoid, madreporite, and the unpaired 

 about forty-five days old, from the oral .side (after Th^el). spicule which, in the o\Aex Pluteus, 

 ant, Anterior unpaired^ ambulacrum ; post, posterior un- arose in its immediate neighbour- 

 ,.aired interan>bulacruni ; 1, tentacle ; 2, spue.s ; 3, spluer- j^^^^ changing into a lattice-like 

 idiam their niches; 4, pieces of the masticatory apparatus: i 



5, teeth ; 6, oral integument, the mouth is not yet formed ; Plate, becomes the madreporitlC 

 7, radial skeletal platps ; S, inteiradial .skeletal plates. basal. Four other plates, which 



arise over the right enterocoel 

 of the larva, liecome the other basals. In their centre the dorso-central is soon 

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

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

 lacral plates appear, with the rudiments of the spines and the sphseridia. botli 

 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 in each interiadins ; 

 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 enterociel, of the hydroc(el {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. 



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 HolotJmrioidea and the Echin- 



