VIII 



ECHINODEmiATA-~(Jj\'TOGENY 



521 



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

 hydro-enteroca'l from the archenteron. 



From the verj- commencement of the formation of tlie liydro-enterocad, the three 

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

 to be marked off' by constrictions, which become more distinct. All three, especially 

 the middle section (the stomach intestine), begin to widen out like vesicles. The 

 blastopore 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-enterocad has been constricted off, the new 

 blind end of the intestine bends ventrally 

 and soon meets a small invagination of the 

 ectoderm of the depressed ventral surface. 

 These break through into one another, and 

 the larval mouth thus enters into open 

 communication with the lumen of the 

 fore-gut. 



During the latter part of the first larval 

 stage, contractile fibres appear in the wall 

 of the oesophagus, and enable this section 

 of the intestine to contract strongly. 



Of the two hydrocoel vesicles lying at 

 the sides of the posterior portion 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 supported 

 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 

 posterior ventral arms, an ectodermal in- 

 vagination appears (Fig. 419, 3) ; this 

 sinks into the blastoc(el in the shape of a 



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

 into the young Echinodenn. 



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

 anterior ventral arms continue to grow (cf. 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 wliioh 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 posterior 

 portion of the oesophagus, 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 posterior 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 



Fig. 418. — Eohinooyamus pusillus, tlie 

 same larva from the left side (after Th^el). 

 1, The first larval arms ; 2, rudiment of the 

 larval mouth; 3, ectoderm; 4, liydro-enterocoel 

 rudiment ; 5, archenteron ; 6, blastopore ; 

 7, larval skeleton. 



