Echinoderma. 



Holothuria and Cucumaria, is in accordance with the views of Semon and the Sa- 

 rasins, who regard the apodal forms as the most primitive [see Bericht for 1888 

 Ech. p 3, 4]. 



Hamann thinks that the Holothurians are degenerate forms derived from the 

 Urchins which had Starfish-like ancestors ; while the Crinoids are the most highly 

 organised Echinoderms. He adopts Loven's view of the houiologies of the apical 

 system, as modified by Carpenter, and rejects Semon's Pentactula theory with its 

 fundamental assumption that the primary tentacles of Holothurians are homolo- 

 gous with the radial water-vessels of other Echinoderms [see Bericht for 1888 

 Ech. p 3]. 



Bury has studied the early relations of the coelomic cavities in Echinoderm 

 larvae. Young Plutei , both of Ophiurids and of Echinids , have two pairs of 

 enterocoels metamerically arranged, the left anterior one opening externally by the 

 water-pore. This is probably the most primitive condition for all Echiuoderms. 

 The hydrocoel is formed subsequently and is probably therefore of later phylo- 

 genetic origin. In Ophiurids it seems to be derived from the left posterior entero- 

 coel, but in Echinids, Asterids, and Crinoids from the left anterior. In most 

 Asterid larvae the two cavities of each side are united, but in one Bipinnaria 

 they are separate on the left, while the two anterior ones do not unite into one 

 common cavity occupying the pre-oral lobe, as is usually the case. The hydrocoel 

 and water-pore both open into the left anterior cavity, as is also the case in Crinoids 

 in which this cavity occupies a median position, that on the right having disappeared. 

 The right cavity is also lost in Holothurians, while the left one is very small, 

 opening both externally and into the hydrocoel ; this therefore, in the larvae of 

 all classes, only communicates indirectly with the exterior, the current passing 

 through the water-pore being probably exhalent. In Holothurians and Echinids 

 the primitive opening between the anterior enterocoel and the hydrocoel lengthens 

 out to form the water-tube (stone-canal). This communication never exists in 

 Ophiurids and closes early in Crinoids, but in Asterids it persists for a time. In 

 all three groups the water-tube is a new development, arising as an outgrowth 

 of the hydrocoel after it has become lobed, and opening into the anterior entero- 

 coel beside the water-pore. But in Asterids it is not in the same interradius as 

 the primitive connection of the two cavities, and is probably therefore a secondary 

 structure, belonging to a comparatively late phylogenetic stage. - - The terminal 

 plates of Ophiurids and Asterids are developed round the left enterocoel , while 

 the radials belong to the right one. B. adopts Carpenter's modification of Loven's 

 views respecting the homologies of the Echinoderm apical system, and points out 

 that it is possible to establish a typical bilateral form from which all the conditions 

 found in existing larvae may have been derived. It would have five radial and 

 five interradial plates on each side, as shown in the following table. 



The ocular plates of 



Urchins are perhaps 



Terminals and not 



Radials. 



Both in Ophiurid and in Asterid larvae the plates on each side may be regarded 

 as forming a longitudinal series dorsal to the gut, the whole arrangement being 

 strongly suggestive of segmentation ; and if the radii be marked by the terminal 

 plates, mouth, anus, and water- pore all occupy the same interradius, as is the 

 case in Antedon and probably also in Urchins. This interradius contains the pre- 



