ECUINODERMATA— ONTOGENY 



521 



certain species this division seems to take iilace as early as the constriction of the 

 hydro-enteroctel from tlie archenteron. 



From tlie very commencement of the formation of the liydro-enteroccel, tlie three 

 sections of the intestinal canal, the hind-gut, the mid-gut, and the oesophagus, begin 

 to be marked off l)y constrictions, wliicli bceonie more distinct. All three, especially 

 the middle section (the stomach intestine), Ix'gin to widen out like vesicles. The 

 ]>lastoi)ore having shifted, as larval anus, far from the vegetative pole ventrally to 

 the centre of the anal area, the hind-gut is drawn up and forms an angle with the 

 mid-gut (Fig. 418\ After the hydro-enterociel has been constricted off, the new 

 blind end of the intestine bends ventrally 

 and soon meets a small invagination of the 

 ectoderm of the dejiressed ventral surface. 

 These break through into one another, and 

 the larval mouth thus enters into open 

 communication witli tlie lumen of the 

 fore-gut. 



During the latter part of the first larval 

 stage, contractile fibres appear in the wall 

 of the I esophagus, and enable this section 

 of the intestine to contract strongly. 



Of the two hydroccel vesicles lying at 

 the sides of the posterior jwrtion of the 

 fore-gut, that on the left opens outward 

 by a water pore at the centre of the dorsal 

 surface. 



Second larval stage. — The two posterior 

 dorsal arms grow out. They are sui)portcd 

 by the rod-like processes of two new cal- 

 careous bodies, which have appeared in the 

 mesenchyme. On the left side, in the 

 angle between the posterior dorsal and the 

 jTOsterior ventral arms, an ectodermal in- 

 vagination apiiears (Fig. 419, 3) ; this 

 sinks into the blastoccel in the shape of a 



flask. This invagination plays an important part in the transformation of the larva 

 into the young Eehinoderm. 



Third stage (fully-grown Pluteus larva). — The two anterior dorsal and the two 

 anterior ventral arms continue to grow {rf. Figs. 400 and 401, p. 509). On the dorsal 

 side, a fifth unpaired calcareous spicule arises, and, in the immediate neighbourhood 

 of the water pore, sends off processes, two of which enter the anterior dorsal arms 

 and support them. The body has shortened, and its posterior region has become 

 rounded off". 



Further differentiation of the hydro-enterocoel.^ — We resume the description of 

 this organ from the stage in which it consisted of two lateral vesicles applied to the 

 intestine. Each vesicle now becomes divided by a constriction into an anterior 

 and a posterior vesicle. The two anterior vesicles lie at the sides of the jtosterior 

 portion of the cesophagus, the two posterior at the sides of the stomach intestine. 

 The left anterior vesicle opens outward through a water pore ; the other three are in 

 no way connected with the future water vascular system ; they are distinguished as 

 the right anterior, right posterior, and left ])osterior enterocoel vesicles. Some- 

 what later, three vesicles are seen on the left side (Fig. 419, 2, 4, 5). The anterior 

 and middle vesicles are in communication, whereas the posterior is distinct, and 

 becomes applied to the middle vesicle, assuming a crescent shape. The left 



^r 



post 



Fig. 41S. — Echinocyamus pusillus, the 

 same larva fmni tlie lelt side (afn-i- Thdel). 

 1, The fir.st larval anus; 2, rudiment of tlie 

 larval mouth ; 3, ectoderm ; 4, hydro-euteroccel 

 rudiment ; 5, archenteron ; (i, blastopore ; 

 7, larval skeleton. 



