MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES, 87 



It is interesting to note tbc wonderfnl coordination of locomotor movements immediately 

 following tlie ain|)ntation of tlirce of tlie amis. In this case if tlic nerve ring lias been uninjured 

 oue of the remaining arms takes up the part of guiding and balancing, while the other strokes 

 first on one side then on the other (text figs. 3 and 4). 



^^■hen the cential nerve ring is cut at any point the coordination in movement is impaired, and 

 when cut in tive places, between the arms, it is lost entirely. 



When placed on its aboral surface an ophiuran (juickly turns over. The method u.sed is quite 

 definite; two adjacent arms straighten out so tiiat together they form a straight line. On these 

 arms as an axis the body revolves, being i>ushed over by the three reniaiuing arms, but mostly by 

 the median one of the three. 



EARLY STAGES. 



The mature eggs are opa(|ue and vary in color from an olive green to an orange yellow. 

 Those of the same individual, however, are constant in their coloration. Until ([uite well 

 developed the larva' retain the color which was on the eggs at the time they were laid. 



For echinodcrms the eggs are very large, being 0.3 millimeter in diameter. 



Soon after they are fertilized the eggs throw olf two membranes, the first of which is much 

 thicker than the second. 



When first laid and during their early develoimient the eggs float, but when their cilia are 

 formed the larva' are able to swim below the surface. 



As I did not know that any sjiecial interest would be found in the life history of the species, 

 I did not carefully observe the early stages while living, nor ineserve material for future study, 

 and as I have stated elsewhere, all later attempts to get other material were unsuccessful. 



This makes it necessary to begin this paper with the description of a late gastrula in which 

 the first iiaii' of enteroco'les have already begun to form as lateral ]iouclies from the anterior free 

 end of the archenteron (figs. 1-3). 



Larvae in this stage of development will be designated as "A.'' 



SXACiE "A," 36 IIOUKS Olii. 

 (Figures 1, 2, aiitl 3.) 



At the age of 3G hours the larv;B swim actively, they being uniformly covered with cilia 



(hg. 1). 



The shape of the larvae is an oval, the length being to the shorter diameter as L' is to 1. 



The animal or anterior jiole is slightly more pointed than the posterior vegetative one. The 

 ventral surface is distinguished by the jiresence of the blastoiiore, which latter has been pushed 

 from its posterior position to a ventral one by the rapid growth of the ectoderm of the dorsal 

 surface of the larva. 



An afiical jilate of taller cells is i)resent at the anterior end, but 1 could not see that the cilia 

 at this point were any longer than those which ('over the other parts of the larva (fig. 3, ap). 



From the blastopore, through which its cavity opens to the exterior, a large archenteron 

 projects forward into the blastocd'le. 



The remainder of the blastoco'le, not taken up by the archenteron or its pouches, is filled with 

 a close network of mesenchyme cells. This mesenchyme tissue is shown in fig. 3, mes, which is a 

 longitudinal sagittal section of "A." 



From the anterior free end of the archenteron a large pouch is in process of being cut <»lf. 

 This pouch projects to the right and left as horn-like ])rocesses, which latter are to be considered 

 the rudiments of the right and left anterior enteroco-les (fig. 2, aer and ael). 



As to the method of gastrulation 1 can not at present speak from observation on larva3 in 

 which it is just taking place, but from a study of the stage now under consideration some idea can 

 be gotten as to how it has proceeded. In figs. 1 and 3 we see a cellular plug (cp) protruding from 

 the blastopore and also extending far into the archenteric cavity. In some cases it extends cvi-n 

 into the enterocu-le i)on(h. The contour of this cellular mass is ragged, which is also true of both 

 the outer and inner surlaces of the wall of the archenteron and the inner surface of the ectoderm. 



These facts seem to indicate that gastrulation does not take i>lace by invagination, as is usual 

 in echiuoderms, but that the larva before gastrulation is a solid, plauula-like afi'air, and later the 



