Chap.V. plan of DEVELOPMENT OF ECHINODERMS. 61 



or an Ophiuran, is developed in exactly the same spot on the sides of the stomach, 

 \ipon the outer surface of opposite water-tubes, one of them forming the actinal, the 

 other the abactinal, surface of the future Echinoderm. The hypothetical form of 

 Huxley, is indeed one which has never been observed, as in all larvae of Echinoderms 

 the mouth and anus are always on the same side, — viz. : on the lower surface of the 

 larva. It is only during the first few days, after hatching from the egg, that the 

 so-called mouth is placed at one end ; but this, however, is not observed beyond the 

 time when this opening performs the double function of mouth and anus, and leads 

 into a very short digestive cavity. By the time the true mouth begins to be 

 formed, the future anus, which has served the purpose of mouth thus far, has already 

 changed its position to the lower side. The mouth is, in fact, never formed at one 

 extremity, but always in the centre of the lower surface, some time after the anus, 

 or temporary mouth. This has been demonstrated by Krohn and myself, with refer- 

 ence to the Echinus larvse, and I trust that the preceding pages have shown it to 

 be also the case with our common Starfish. The division into rings, of what 

 Miiller calls the Wurmformige Asteridenlarve, is only an optical delusion, due to the 

 lines formed upon the abactinal surface during the closing of the pentagon. 



The radical difference in the mode of formation of the oesophagus, stomach, and 

 intestine, in the Echinoderm larvse, as compared with the larvse of Annelids, a 

 number of which I have examined myself, and among them those most resembling 

 Echinoderm larvas, will, perhaps, be the strongest proof that they do not belong to 

 one and the same type. The digestive cavity of Annelid larvse is formed by the 

 liquefaction of the interior of the larva, while in the Echinoderm larvse the diges- 

 tive cavity is formed by the bending in of the outer wall of the larva itself. The 

 appendages of Annelids, which give them their superficial resemblance to the larvse 

 of Echinoderms, are those surrounding the mouth, while the principal appendages 

 of the Echini and Starfish larvse are developed from the vibratile chord which is 

 developed round the anus. There is nothing more characteristic of the Echinoderms 

 among Eadiates than the isolation of the digestive cavity by means of distinct walls. 

 This feature is so strongly marked that a larva can be recognized as an Echinoderm 

 larva before its radiate characters are developed. It is only later that the circular 

 tube, the water-system, is formed, and, still later, that the ciliary appendages, which 

 have nothing to do with the formation of the Echinoderm, make their appearance 

 long after the first rudiments of the Echinoderm (the water-tubes) are present. 



It seems to me that the different modes of development in Holothurians, Echini, 

 true Starfishes, Ophiurans, and Crinoids, different as they are apparently, may easily 

 be reduced to a single type. We have in Ophiurans two different modes of 

 development, one by means of the Pluteus, the other by means of the mode 

 of development observed by Krohn and Schultze. We have two similar modes 



