THE AFFINITIES OF THE ECHINODERMATA. 505 



cessively developed. The entire zone of the calyx, which is 

 occupied by the origins of the arms, at the same time widens, 

 so that the oral plates, which remain round the mouth, and 

 the basal plates, which encircle the stem, become widely sep- 

 arated. The intestine grows out as a diverticulum of the 

 alimentary cavity, and opens on an interradial elevation of the 

 calyx, in which an anal plate is developed. The young Echi- 

 noderm has now passed into the stalked Pentacrinoid stage. 



In Comatula, the oral and anal plates disappear altogether, 

 and the basals, coalescing into the rosette, are hidden by 

 the first radials, on the one hand, and the centro-dorsal tuber- 

 cle, which represents coalesced joints of the stem, on the 

 other. The arms bifurcate and acquire their pinnules ; and 

 the calyx, with its appendages, eventually becomes detached 

 from its stem as a free Comatula. In the existing stalked 

 Crinoids, such as JPentacrinus, on the other hand, the seg- 

 ments of the stem acquire whorls of cirri, at intervals, and no 

 such modification of the uppermost segments into a centro- 

 dorsal tubercle takes place. 



On comparing the facts of structure and development 

 which have now been ascertained in the five existing groups 

 of the Echinodermata, it is obvious that they are modifications 

 of one fundamental plan. The segmented vitellus gives rise 

 to a ciliated morula, and this, by a process of invagination, is 

 converted into a gastrula, the blastopore of which usually be- 

 comes the anus. A mouth and gullet are added, as new for- 

 mations, by invagination of the epiblast. The embryo normally 

 becomes a free EchinopaBdium, which has a complete alimen- 

 tary canal, and is bilaterally symmetrical. The cilia of its 

 ectoderm dispose themselves, in one or more bands, which 

 surround the body ; and, while retaining a bilateral sym- 

 metry, become variously modified. In the Holotharidea, As- 

 teridea, and Crinoidea, the larva is vermiform, and has no 

 skeleton ; in the Echinidea and the Ophiuridea it becomes 

 pluteiform, and develops a special spicular skeleton. 



If an Echinopaedium were to attain reproductive organs, 

 and reproduce its kind, I think that it cannot be doubted that 

 its nearest allies would be found among the Turbellaria, the 

 Rotifera, the Gephyrea, and the Enter opneusta. 1 But that 



1 In a report upon the " Researches of Prof. Muller into the Anatomy and 

 Development of the Echinoderms," published in the Annals of Natural His- 

 tory for July, 1851, I drew attention to the affinities of the Echinoderms with 

 the Worms'; and in a paper on Lacinularia socialis, read before the Micro- 



22 



