Echinoderma. 



from the Pentactula stage, indicate true homologies. The importance of the water- 

 vascular system is also shown by its share in regeneration after fracture. It is pro- 

 bably excretory in its primitive character, and as it became locomotive, the circu- 

 latory function was transferred to the blood-vascular system, the constant relation 

 of which to the mesenchym is also primitive. The Pentactula-stage of Antedon 

 has a nerve-ring around the oral funnel, the histological characters of which are 

 identical with those of the^ynajote-larva, and the general relations of this nervous 

 system in the Pentactula are wholly palingenetic. It is least modified in Asterids 

 and Crinoids, remaining ectodermal, but in other respects the former are farther 

 from the primitive type than the remaining Echinozoa. As regards the geni- 

 talia, the Holothurians with paired organs at the sides of the dorsal mesentery 

 are nearest the primitive bilateral type ; while in Echinids and Ophiurids they take 

 on a radiate arrangement, and extend into the arms of Asterids and Crinoids. The 

 genital plates of Urchins are not homologous either with the orals or with the basals 

 of Crinoids, as asserted by Ludwig and Carpenter respectively [see Bericht for 

 1880 I p 249]. The Pentactea may have had a calcareous support for the primary 

 tentacles, asinthe<Synaj0ta-larva, and also an irregular integumental skeleton which 

 developed along independent lines in the different classes, always, however, taking 

 a more or less radiate arrangement which is based upon that of the water-vascular 

 system. - - The Dipleuru la-larva is to a great extent a palingenetic form, as its 

 bilateral symmetry which becomes permanently indicated by the dorsal mesentery 

 is more than a mere adaptation to its mode of life . It represents the commingling ' 

 of two phylogenetic stages , viz. bilateral animals with an extensive mesenchym 

 but no coelorn nor hydrocoel, and bilateral animals with a paired enterocoel. The 

 change from bilateral to radial structure was probably induced by assumption of a 

 fixed mode of life. 



The cousins Sarasin ( 3 ) point out that the Echinothuridae present many 

 structural features which are characteristic of the young stages of other Euechinoidea, 

 and are closely related to the Palaeechinidae. The latter group is probably derived 

 from mailed Holothurian ancestors; while among the Holothurians the Pedata pass 

 through an Elasipod stage of development and are probably derived from the 

 Apoda through the Elasipoda. In like manner young Echini with an imperfectly 

 developed skeleton and only 5 primary tentacles are in the Holothurian stage. The 

 Echini cannot be derived from the Criuoids on account of resemblances in the 

 apical system, as the calyx is so irregular in the earliest Crinoids. The latter 

 group is more probably descended from the Cystids, as they pass through a Cystid 

 phase of development; and their stem is possibly derived by modification from a 

 conically extended basal pole , as indicated by forms like Glyptosphaerites and Ca- 

 ryocystites. The Cystideans themselves have many resemblances to the Holothu- 

 rians, viz. an irregularly plated moveable perisome, the position of the genital 

 pore and often that of the anus (Psolidae) . The arms of the Cystids perhaps arose 

 as outgrowths at the ends of the water-vessels of the body , indications of which 

 remain in Glyptosphaerites, Another difficulty in the comparison of the apical systems 

 in Crinoids and Urchins is the absence of a subanal plate in early stages of the 

 Echinothuridae , Cidaridae and Diadematidae , the three oldest groups of living 

 regular Urchins. But it occurs in adult Salenitlae and in the young of other regular 

 Urchins which are thus to be regarded as descendants of the Salenidae (Saleni- 

 g o n a e] , while those without an embryonic subanal plate may be called Praesale- 

 niadae. - - Ophiurids and Asterids, like the Urchins, can probably be derived 

 from the Holothurians, and among the latter the apodal forms are probably most 

 primitive. Through them the Echinoderma come into relation with the uusegmen- 

 ted worms, as they do in the characters of their nephridial system. 



