32 EMBEYOLOGICAL MONOGIIAPHS. 



The digestive cavity itself shows the fust trace of dilfcreiitiation iuto intestine, stomach, and oesoph- 

 agus. 



2"2. Prolile view of an embryo, somewhat older than fig. "21, at the beginning of tlie fifth day ; the intestine, c, 

 stomach, d, and cesophagus, are well separated ; the anal vibratile cord (c") bulges out considerably 

 beyond the depression in the vi.-ntral side of the Pluteus. 



23. Embryo at the end of the fifth day, seen from the mouth siiie ; the water-tubes (ii>, w') are only slightl)' con- 

 nected with the digestive cavity ; they also show a difference in size. The original limestone rod has given 

 off a shoot, the rod of a new arm to be developed at v. 



2-t. Fig. 23 seen from the anal extremity, to show the great change of form which has taken place from the early 

 cylindrical shape of the embryo. 



25. A profile view of lig. 23. The embryo has become pear-shaped, the cesoph.agus has bent over to reach the 



ventral side ; the anal opening is also somewhat ventrally placed ; the depression at m, where the new 

 montli is to be formed, is in contact with the ossophagus ; the anal and oral vibratile cords have increased 

 in prominence. 



26. A profile view of an enjbryo, taken at the beginning of the seventh day. The mouth, m, is open ; the water- 



tube !(;' reaches nearly to the dorsal surface. The currents, which previous to this stage had carried the 

 food through the only opening, a, into the digestive cavity as far as o, and then were reversed to eject the 

 digested matter, now come in through the mouth, m, pass through the a-sophagus, o, rotate about in the 

 stomach, rf, and pass out through the first-formed opening, the anus, a, which is liereafter only used to 

 eject the food. 



27. A Pluteus at the end of the eighth day, seen obliquely from the ventral side, to show the course and shape of 



the vibratile cord. 



28. Profile view (actinal) of a stage slightly older than that of fig. 27. 



29. The same as fig. 28, seen from the ventral .side. 



30. Somewhat more advanced Pluteus, to show the changes the vibratile cord has undergone since the stage of 



fig. 27 ; seen obliquely from the ventral side. 



31. A Pluteus during the tenth day, seen in profile, shows the beginning of the small arms c'" and civ. 



32. The same as fig. 31, seen from the mouth side. The arms c' liave been greatly developed ; the differentiation 



of the intestine, c, the stomach, d, and the resophagus, is quite complete. First appearance of the vibratile 

 epaulettes, r". The water-tubes have not yet united, and have not greatly increased in size from the jire- 

 ceding stage. 



33. Profile of a Pluteus during the twenty-third day. The arm <" has increased greatly in length, and a con- 



siderable increase of the vibratile epaulettes is to be noticed. 



34. The same as fig. 33, seen from the dorsal side, to show the relation of the rods of the aims c', c", c'", to 



each other ; the water-tube Jc' shows the first indication of a large tentacular lobe. 



35. Fig. 34, seen somewhat obliquely, in an attitude similar to that of figs. 27 and 30, to show the coimection of 



the different parts of the vibratile cord. 

 3G. A much more advanced Pluteus, fished up from the surface. The rods extending into the arms are made up of 

 three sets of rods united by short transverse bars ; the whole oral extremity of the larval body has greatly 

 lengthened ; the arms c'" and fiv are longer than in the preceding stages, c', c", and c'" being nearly of 

 equal length ; the arras show a tendency to a paired arrangement of e', e", and c'", c'v. Additional ten- 

 tacular lobes liave been formed in the water-tube ^v', and the water-tubes have become united in the oral 

 extrenuty beyond the saclike pouch of the mouth of the Pluteus. 



37. Fully developed Platens of SlrOHgijloccntrotu.i, in which the young Sea-urchin has alreaily encroached .some- 



what on the anal extremity ; its spines are quite well marked ; the vibratile epaulettes have acquired a 

 gi-eat size ; two very prominent spots, s', s', in the arms <■', c". At the base of the oral extremity of the 

 mouth pouch a rudimentary appendage, /, appears ; this is perhaps the homologue of the brachiolarian 

 appendag<-s of the Brachiolaria of Asteracanthion. The arras c', t", and c'", t-iVj mv. now of nearly equal 

 length, and ananged in pairs. 



38. Fig. 37 seen from the oral extremity of the Pluteus. 



39. Profile view of fig. 37. 



40. A young Echinus, iniTuediately after the resorption of the Pluteus, seen from the abactinal side. The anal 



opeiung cannot be traced in the youngest specimens, tlio\igh it is. very apparent in somewhat nmre advanced 

 stages (a, lig. 43). 



41. A young Ecliinus, somewhat more advanced tluan the stage of fig. 40, seen from the actinal side ; the tenta- 



cles have become more slender ; the odd tentacle V especially, wherk fully extended, more than equals the 

 diameter of the test ; the interambulacral spines of this side are nearly as long as the diameter of the test. 

 The actinal system is not well separated from the coronal test. The two tentacles nearest the actino- 

 stome are remarkable for the great development of the sucking disk. 



42. The .same as fig. 41, seen from the abactinal side ; the .spines of the abactinal area are remarkable for their 



fan-shaped spiny extremities. 



