8 EMBRYOLOGY OF THE STARFISH. Part I. 



which can best be compared to the embryos of other Radiates; for there is as yet, 

 nothing of the complication hereafter introduced in the subject by the development 

 of bilateral parts, obscuring the plan upon which the embryo is built. It is an embryo 

 closely resembling those of the other Radiates, in which, however, the class-characters, 

 distinguishing it from the embryos of the other classes of the type, are already de- 

 veloped beyond question. In the young Polyps, the earliest appearance of the class- 

 characters is denoted by the presence of a few radiating partitions, dividing the cavity 

 of the embryo into distinct chambers. In the Acalephs, in the most rudimentary 

 stages, we already find the chymiferous tubes pushing their way through the sphero- 

 some; while, in our larvae, the echinodermoid class-character, that of having distinct 

 walls, forming the different organs, is already plainly visible from the mode of forma- 

 tion of this digestive cavity. What unites all these embryos in one great type is, 

 that we have in them all, an axis around which are arranged the different elements 

 of which they are composed. Our young Echinoderm, in this condition (PI. I. fflgs. 

 23-28), can be strictly homologized with the earlier stages of a Polyp at the time 

 when the digestive cavity is first formed, before the appearance of the partitions ; and 

 to an acalephian embryo, where the digestive cavity alone is developed, previous to 

 the pushing of the chymiferous tubes through the gelatinous mass. The stages subse- 

 quent to the condition of the embryo here described, represented in PL I. Fig. 24, not 

 having been traced very carefully by previous observers, we have not had before us the 

 means of forming a true conception of the mode of development of the Echinoderms ; 

 for to obtain a clear and precise idea of the functions of those problematic bodies which 

 have puzzled Miiller during the whole of his investigations, it is necessary to follow, step 



as they have been figured by Derbes, agree in the though he has traced the development from the egg 



main points with what has been observed of the of several Echinoderm larvae, yet he has not given 



earlier stages of our Amorican Echinus larvae (Tox- us as detailed descriptions and figures of the earlier 



opneustes drobachiensis). With the exception, how- stages, as of those which were more advanced, and 



ever, that Derbes, not having followed all the inter- says simply, that in the main points his observations 



mediate stages between his figures 15 and 16 in coincided with those of Krohn and Derbes. Krohn, 



the Annales des Sciences Naturelles for 1847, did who has artificially fecundated E. lividus, gives us in 



not see the transformations the digestive cavity un- his figures some of the missing links in the chain 



dergoes, and committed, therefore, the very natural of the observations of Derbes, and shows distinctly 



mistake of supposing that the first formed opening, for Echinus lividus, that the first-formed opening 



which we have described as a mouth, retained the becomes the anus eventually, and in what way this 



same function afterwards. He, however, correctly is brought about by the bending of the bottom of 



noticed the separation of the three cavities, the oesoph- the digestive cavity towards one side of the larva, 



agus, the stomach, and the alimentary canal, into as is the case in our Starfish, and the formation at 



which this primary cavity is gradually differentiated, that point of a second opening, which becomes the 



and has given a correct description of their relation true mouth, while the first -formed opening hence- 



to each other. Miiller has taken up this same subject forth assumes the function of an anus. 

 more where Krohn and Derbes have left it, and al- 



