MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 87 



It is interesting to note the wonderful coordination of locomotor movements immediately 

 following tlie amputation of three of the arms. In this case if the. nerve ring has been uninjured 

 one 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 flgs. 3 and 4). 



When the central nerve ring is cut at any point the coordination in movement is impaired, and 

 when cut in h' ve places, between the arms, it is lost entirely. 



When placed on its aboral surface an opliiuran quickly turns over. The method used is quite 

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

 arms as an axis the body revolves, being pushed over by the three remaining arms, but mostly by 

 the median one of the three. 



EARLY STAGES. 



The mature eggs are opaque and vary in color from an olive greoi to an orange yellow. 

 Those of the same individual, however, are constant in their coloration. Until quite well 

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



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



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

 thicker than the second. 



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

 formed the larv;e are able to swim below the surface. 



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

 I did not carefully observe the early stages while living, nor preserve 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 gastrnla in which 

 the first pair of enterocreles have already begun to form as lateral pouches from, the anterior free 

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



Larv;e in this stage of development will be designated as "A." 



STAGE " A," 36 HOURS OLD. 

 (Figures 1, 2, ami 3.) 



At the age of 36 hours the larvae swim actively, they being uniformly covered -with cilia 



(fig. 1). 



The shape of the larva? is an oval, the length being to the shorter diameter as 2 is to 1. 



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

 ventral surface is distinguished by the presence of the blastopore, 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 apical plate of taller cells is present at the anterior end, but I could not see that the cilia 

 at this point were any longer than those which cover 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 blastociele. 



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

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

 longitudinal sagittal section of "A." 



From the auterior free end of the archenteron a large pouch is in process of being cut oil'. 

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

 the rudiments of the right and left auterior euterocojles (fig. 2, aer and ael). 



As to the method of gastrulation 1 can not at present speak from observation on larva? 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 even 

 into the enteroccele pouch. The contour of this cellular mass is ragged, which is also true of both 

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



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

 in eehiuoderuis, but that the larva before gastrulatiou is a solid, plaiiula-like affair, and later the 



