OUTLINE OF THE YOUNG STARFISH. 49 



Memoir on Sea-Urchins,* where he has most thoroughly proved the homol- 

 ogy of the basal and radial plates of Crinoids with their corresponding 

 plates, still readily to be traced in the young Starfish, and with their 

 homologies in the apical system of Echini. 



An admirable paper by Selenka in the Zeits. f. Wiss. Zool. (June, 1876) 

 gives us a complete history of the development of Holothuria, showing 

 an entire agreement in its general features with the Embryology of other 

 Echinoderms in the mode of formation of the water-system as diverticulum 

 for the alimentary canal, forming eventually (as in the Starfish) the cir- 

 cular canal with the ambulacral system.] 



Change of Outline of the young Starfish. — With advancing age, the outline 

 of the young Starfish is greatly modified ; at first, when the actinal and 

 abactinal areas are not yet closed, while the larval appendages are still 

 visible on the lower side of the young Starfish (PI. VI. Figs. 1, 2), im- 

 mediately after the larval appendages have disappeared, and the surfaces 

 of the actinal and abactinal areas are brought nearer together (PI. VI. 

 Figs. 3, 4), it is hardly more than an irregular pentagon, with slightly 

 convex sides, and small rounded notches cut in at the angles (PI. VI. 

 Figs. 3, 4). These notches become deeper, the arms of the Starfish as- 

 sume more the appearance of a Greek cross (PI. VI. Figs. 6, 7) ; the sides 

 of the rays are strongly concave, and the concavity is increased with the 

 development of the spines to such a degree that the extremity of the 

 ray is almost twice as broad as its base (PI. VI. Figs. 10, 11, 12). The 

 outline of the inner wall of the disk can be easily seen through the lime- 

 stone network. The pentagonal form, so different from that of the 

 adult, is still less like it when seen in profile (PI. VII. Fig. 2). The abac- 

 tinal area rises like a high, rounded cone, supported upon the spines {p) 

 of the edge of the disk ; the tentacles project far beyond the edge on 

 every side (PI. VII. Fig. 2). In fact, the regular rows of spines, their great 

 size, the convexity of the disk, are features so unlike our usual concep- 

 tion of a Starfish that, without closer examination, one would readily mis- 

 take this Echinoderm, at first sight, for a young Sea-urchin, like the flat, 

 conical Ecliinocidaris. 



The tentacles are longer than the rays, extending far beyond the edge 

 in front and on the sides. The pairs of tentacles move in every direc- 



* Kongl. Svenska Vets. Akad. Hamll. XI. No. 7. Etudes sur les Ecliinoidees par S. Loven. 

 Stockholm, 1874. 



