8 BULLETIN OF THE 



starfish antedate all plates on the actinal surface. With the formation 

 of the eleven plates of the abactinal hemisome, how fares it witli the 

 actinal ? "What plates have been added tu this portion of the body 1 



If we so place the brachiolaria that the side opposite that already 

 described (abactinal) is made to face the observer, it will be seen that 

 the future circular water tube on this side (actinal) has the form of an 

 elongated U-shaped tube, with five blind extensions (culs-de-sac, fig. 3, 

 rw). If we focus the microscope down to the plates below, which lie on 

 the actinal surface, on the opposite side of the stomach, it will be seen 

 that each of these blind extensions of the tube corresponds roughly with 

 a radius of the starfish. Like the disk, the circular tube has likewise 

 a brachiolarian notch, but it is unclosed as yet. This tube and its 

 extensions are the water-vascular tubes of the future starfish ; the un- 

 closed U-shaped portion, the circular tube ; tlio prolongations, or culs- 

 de-sac, are the beginnings of the median actinal water vessel * of the 

 starfish rays. 



One remarkable fact in regard to the relation of the system of vessels 

 to the abactinal plates may be mentioned. The middle of the region 

 on the periphery of which the circular tube of the actinal vessels lies, 

 or the mouth, does not coincide with and is not opposite to the dorso- 

 central, or the central plate of the abactinal hemisome, but is more 

 nearly opposite the point of origin from the circular vessel of the madre- 

 poric tube. This dislocation has been brought about by the fact that 

 the fifth terminal has grown to be much larger than the first terminal. 

 jNIoreover, the indentation which separates the first terminal from the 

 second, is in early stages quite obscure, for some unknown reason. The 

 same is true of the circular tube which near the fifth terminal, t^, is 

 well formed, while near the first terminal, t^, it is as yet unclosed. The 

 first radial median projection is half formed. This condition is due in 

 part to the fiict that the plane of the water ring is not yet quite parallel 

 wnth that of the terminals, as explained above, on the abactinal hemi- 

 some. The unequal development would seem to indicate that the fifth 

 terminal is the first to form, and that the first terminal is the latest 



* The primary form of the circular bloodvessel of Asterias can be seen ia 

 Plate I. fig. 3, cbr. It occupies the same position as the homologous structure in 

 Asterina, and skirts the walls of the opening of the cesophagus into the stomach 

 anil the primitive water vessel. Extensions or radial vessels extending from this 

 into the arms were not noticed, and the central tube, ncr, is as ytt not closed in 

 or united at the brachiolarian notch. The early-formed nervous rinsr lies on this 

 region of the starfish, on the bloodvessel; but its tissue is with difficulty distin- 

 guished from that of the bloodvessel upon which it lies. 



