^98. 



THE ECIIINODERMATA. 



99 



The first condition of every Ecliinoderm is the 

 6ame, — an oval, ciliated body, resembling an in- 

 fusorial animalcule, and without external organs, 

 or distinction of parts. This is the starting-point, 

 -and upon it succeed variations accomiing to the 

 •different famiUes. Upon this ciUated body are 

 developed, at one part, peduncles for its attachment 

 to other bodies, while the rest of the germ increases 

 in size, and assumes a star-fish form. 



The larvae thus formed may be divided into two 

 groups : 



1. Those of the Ophiuridae and Echinidae. 



2. Those of the Asteroidae and Ilolothuridae. 

 The first are somewhat hemispherical bodies, 



•with one edge of their truncated side prolonged 

 into a single flat and wide process, which carries 

 the mouth and oesophagus ; while from the oppo- 

 site extremity project rods, of four, eight or more 

 in number, and which form the internal skeleton. 

 (See Ueb. d. Ophiurenlarven d. Adriat. Meeres. 

 Taf. I. II.) These larvaj have a globular stomach 

 in then- hemispherical portion, and from which pro- 

 ceeds a short intestine terminating in a circular 

 ■anus. They have, moreover, a ciliated fringe, 

 •which consists of a ridge covered with large cilia, 

 passing above the mouth and before the arms, com- 

 pletely encircling the body in an oblique manner. 



With the second group there is no internal cal- 

 careous skeleton, and they form 3IüUer''s Auricu- 

 iaria (of the Ilolothuridae), and Bipinnaria (of the 

 Asteroidae). 



The first of these are concavo-convex bean- 

 shaped bodies, with an Irregular transverse fissure 

 answering to the hilum of the bean, in which the 

 mouth is placed. The margins of this fissure are 

 •ciliated ; the anus opens on the ventral surface. 



The Bipinnaria closely resemble these last, but 

 they have a distinct ciliated circle in front of the 

 mouth ; as they increase in size, the anterior part 

 of their body is covered with long processes, which 

 "vary according to different forms. 



Out of these larvae, all of which have a strictly 

 "bilateral symmetry, the more or less radiate adult 

 Echinoderms are developed by a process which is 

 a sort of internal gemmation. 



The changes and variations of this metamor- 

 phosis I will give in Müller''s own words : 



" 1. The change of the bilateral larva into the 

 Eohinoderm takes place when . the larva yet re- 

 mains an embryo, and is universally covered with 

 •cilia, without a ciliated fringe. A part of the body 

 •of the larva takes on the form of the Echinoderm ; 

 the rest is absorbed by the latter (a part of the 

 Asteroidae, Eckinaster, Asteracanthion^ Sars). 



" 2. The change of the bilateral larva into the 

 Echinoderm takes place when the larva is perfectly 

 organized ; that is, possesses digestive organs and 

 a special ciliated fringe. 



" The Echinoderm is constructed within the Plu- 

 teus like a picture upon its canvas or a piece of 

 embroidery m its frame, and then takes up into 

 itself the digestive organs of the larva. Hereupon, 

 the rest of the larva vanishes (Ophiura, Echinus), 

 or is thrown off {Bipinnaria). 



" 3. The larva changes twice. The first time it 

 passes out of the bilateral type with lateral ciliated 

 fringe into the radial type, and receives, instead of 

 the previous ciliated fringe, new locomotive larval 

 organs, the ciliated rings. Out of this pupa-condi- 

 tion, the Echinoderm is developed, without any part 

 being cast off (Jlolothuria, some Asteroidae). 



"If we call embryonic type the condition ia 

 which the animal leaves the egg, and when the 

 internal organs are not yet developed, we have four 

 stages or types, — the embryonic type, the larval 

 type, the pupa type, and the Echinoderm type. 

 The animal may pass from either of the first three 

 forms into the Echinoderm, or may run through 

 them all." See Ueber. d. Larven u. d. MetamoriJh. 

 d. Holoth. u. Aster, p. 33. See, also, a review of 

 Muller^s researches, by Huxley (.\im. Nat. Hist. 

 VIII. 1851, p. 1), and by Dareste (Ann. d. Sc 

 Nat. XVII. 1852, p. 349). 



These results are highly interesting in both a 

 zoological and a physiological point of view, and I 

 need only suggest their important relations to the 

 doctrine of " alternation of generations." 



In this connection, it may be proper to allude to 

 another point. It is well known that Vogt (Na- 

 turgesch. d. lebend, u. untergegang. Thiere. I. 

 Liefer. 3, p. 25-1) has removed the Bero'id Medusae 

 from the Acalephae to the " Molluscoida," re- 

 garding them bilateral animals. In a private 

 letter from Agassiz, there is a passage bearing 

 du-ectly on this point. He says : " The young 

 Echinoderms are structurally and morphologically 

 homologous with Bero'id Medusae, showing that 

 Beroüds are genuine Radiates, and truly belong to 

 the class of Acalephae, and cannot be referred to 

 the MoUuscoids. These relations will be plain by 

 comparing Taf. I. fig. 6, of Muller's Larven und 

 d. Metamorph, d. Ophiuren und Seeigel. 1848, with 

 the figures of PI. VIII. of Agassiz'' Memoir on the 

 Berold Medusae, in the Mem. of the Amer. Acad, 

 of Arts and Sc. Vol. IV." 



For further writings on the development of the 

 Echinoderms, see Müderes papers, published in 

 his Arch. 1848, p. 113 ; 1849, p. 84, 364 ; 1851, p. 

 1, 272, 353 ; but these papers are all included in 

 his large memoirs already given. See, also, Krohn, 

 Beitrag, zur Entwickelungsgeschichte der Seeigle- 

 larven, 1849, and in Muller's Arch. 18.51, p. 338, 

 344, 368 ; and Desor, Muller's Arch. 1849, p. 79. 

 — Ed. 



