20 EMBRYOLOGICAL MOXOGEAPHS. 



12-19. Asteracanthion flaccida. From dniwings made under the supervision of L. Agassiz, In 1848. 



d. Abaetinal side of tlie young Starfish. 



p. Larval organ (brachiolarian aiijiend.ages). 



t. Terminal tentacles. 



f, I", fi, I*. First, second, third, and fourth pair of tentacles, counting from the terminal tentacle. 



12. I'rolile view of larva, with only the terminal tentacles developed. 



13. The same as fig. 12, seen from the actinal side of the young Starfish. 



14. Somewhat older stage than fig. 12 ; besides the terminal tentacles, t, an additional pair of tentacles, V, has 



been developed. 



15. The .same as fig. 14, seen from the actinal side. 



16. Still older stage, with a second pair of tentacles, I', developed in addition to the t and (". 



17. The same as figure 16, seen from the actinal side. 



18. Still older stage ; the larval organ, p, is nearly resorbcd, and there are now in each arm four pairs of tentacles, 



V-tS, in addition to the terminal tentacle, t. In figs. 14-19, ^'always denotes the last-formed tentacle, /" the 

 last but one ; the outline of the young Starfish becomes more and more pentagonal as it passes through 

 the stages of figs. 13 to 15, and 17. 



19. The young Starfish has five well-formed arms, four pairs of tentacles, /'-/*, a distinct and closed circular water- 



ring. The actinostome of the .Stai'fisli is not yet formed, and the larval organ has not been completely re- 

 sorbed. 



20-23. Pterastcr mililaris. From M. Sars, Oversigt af Morges Echinodernier, 1861, PI. VI. 



a. The five perianal plates. d. Remnant of the larval organ. 



b. Terminal arni-plates. c. Circular water-ring. 



c. Second arm-plates. g. Terminal tentacle. 



t', /", t"K Third, .second, and first pair of ambulacral tentacles. 



20. Young Starfish, seen from the dorsal side ; a, the five perianal plates ; b, the ten arm-plates. 



21. One of the arms, seen from the actinal side ; b, oldest arm-plates ; c, younger arm-jilates ; rf, remnant of larval 



organ ; c, circular water-canal ; /, ambulacral canal ; g, terminal tentacle ; I', I", t'", third, second, and 

 first pairs of ambulacral tentacles. The tentacles are contracted. 



22. One of the arms, of a stage somewhat older than fig. 21, seen from the actinal side, with the tentacles extended. 



23. Fig. 21, seen in profile. 



24-32. Asteracanthioa violairns. From C. Wyville Thomson, On the Embryology of Asteracanthion violaceus, L. 

 Quart. Journ. Microsc. Scieu., I., 1861, PI. VII. 



24. Embryo about four hours after complete segmentation. 



25. Embryo four hours later ; first formation of the larval organ, the so-called peduncle, or brachiolarian arms. 



26. Embryo about nine hours later, with three brachiolarian appendages. 



27. Embryo about twenty-four hours old. 



28. Peduncle and appendages, which have become separated by a natural process of fission from an embryo about 



a week old. 



29. The embryo has become distinctly pentagonal ; the brachiolarian appendages are fully developed. Tliirty-si.'c 



hours after segmentation. 



30. Embryo in which the pentagonal outline of the yoimg Starfish is well defined, with five tentacles ; the brachi- 



olarian appendages are beginning to be resorbed, and are no longer very efficient in assisting locomotion, 

 as in the earlier stages. About eight days after segmentation. 



31. Embryo five weeks after segmentation ; shows the remains of brachiolar appendages, much atrophied ; the ac- 



tinostome of the young Starfish is indicated, and there are three pairs of tentacles in addition to the odd 

 terminal one. 

 31'. Abactinal view of a young Starfish, about twelve days old, showing the arrangement of the limestone plates of 

 that side. 



32. View of the actinal surface to show the arrangi'ment of the aTiibulaeral plates. 



33-42. Echinastcr sanguinnhntiis. From M. Sars, Ueber die Entwickelung der Seesterne. Fragment aus 

 meinen Beitrage zur Fauna von Norvegen. Arcdiiv fiir Naturg., 1844, I., PI. VI. 



a. Brachiolar ajipendages. b. Tubci'cular brai-hiolar appendage. c. Ambulacral tentacles. 



33. Egg just laid, greatly magnified. Chorion colorless ; yolk, brilliant orange-red. 



34. Egg three days old, in which the blastosphere has been formed. 



35. Embryo just after being hatched. 



