MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 95 



McBRiDE (10) and GOTO (5) both agree that iu starfishes the inner perihnemal ring sinus 

 arises from the anterior euteroccele, although they differ as to the method of its formation. In 

 none of the larva; I have is the structure iu question fully formed, but in Stage "F"-a cavity is 

 arising, as an outgrowth from the anterior enterocade in the stone canal interradius, which I 

 take to be the rudiment of the inner perihiemal ring space. It lies to the left of the stone canal 

 near the median sagittal plane of the larva. From the posterior side of the ventral end of the 

 anterior enterocu'le the outgrowth takes its origin, then extending posteriorly until past the water 

 ring it bends over and grows down until its end reaches the nerve ring at a point inside the outer 

 perihiviual sinus. Here the end of the pouch in question begins to spread under the nerve ring 

 in both directions, parallel to the outer periLuemal ring (figs. 24 and 26, ips). This coincides 

 exactly with its method of origin in Asterina gibbosa as described by McBRiDE. 



Although the outer periluemal ring is entirely cut off from the body cavities at this stage, 

 there still remain traces of the interradial pouches which gave rise to it (fig. 29, hip 1-2 and 4-5). 



The hypogastric enteroccele itself has changed very little since Stage "E," but the epigastric 

 has enlarged to such an extent that its edges now meet the edges of the hypogastric and a circular 

 aboral mesentery is formed (figs. 23-29. cm). 



In the water system considerable growth is to be noted in the tentacles, the end and first 

 pair of foot tentacles being capable of protrusion considerably beyond the disk. By means of 

 these tentacles the larva- are able to cling tenaciously to the surfaces of foreign bodies, it requiring 

 a strong jet of water from a pipette to detach them. Special notice was taken to be sure that it 

 was the tentacles and not the larval organ which was used as a means of attachment. 



The second pair of tentacles (buccal tentacles) have acquired their ectoderm in this stage and 

 they protrude, like the other tentacles, over the radial nerves into the radial grooves (figs. 22 

 and 20, t2). 



The axial sinus or ampulla is present in "F," it being that part of the anterior enteroccele 

 which remains after the pouches have been cut oft', which will form the inner perihtemal and part 

 of the outer pcriha-uial systems (figs. 24, 25, and 20, ax sin). 



It will be noted that beside the, ampulla, which is situated anteriorly to the stone canal, there 

 are two other cavities near the stone canal to be accounted for (fig. 20, sin a and sin b). I can 

 not be sure of their origin, but I believe that they also come from the anterior enterocoele. I have 

 distinguished them by the letters a and b, as they are probably the same cavities as those so let- 

 tered by McBuiDE (10) in his figures of Amphiura. 



The cavity McBRiDE has lettered b', and which he thinks represents the degenerated right 

 hydroecele, I have been unable to find in any of my sections. 



The stomach and oesophagus are in a condition the same as we found them in "E;" that is, 

 without luiueii. 



STAGE "G," 5} DAYS OLD. 

 (Figures 31 and 32.) 



Larvre a few hours older than "F " show a decided degeneration in the larval organ (fig. 31, lo), 

 but otherwise the external appearance of the two stages is about the same. 



The grooves caused by the invaginatiou of the nervous system have begun to disappear by 

 the growing together of their edges, and instead of the deep furrows we find a slightly depressed 

 line where the edges of the grooves have met (fig. 28, rg). 



In the nervous system a pair of tentacle nerves has been formed from each radial nerve (fig. 

 31, nl). They grow out laterally from the radial nerves at points proximal to the first pair of foot 

 tentacles, around which latter they grow and to which they belong. No nerves as yet have 

 appeared to supply the buccal tentacles. 



In "F" the buccal tentacles had only begun to curve away from the end of the arm; but in 

 "G" this proximal bending has continued until they curve over the nerve ring and point toward 

 the mouth. 



Beside this change in the water vascular system we find in "G" the first appearance of the 

 rudiments of the polian vesicles. They are four in number and are in the form of small interra- 

 dial pouches growing distally from the convex wall of the water ring (figs. 31 and 32, pv). There 

 is no poliaii vesicle present in this species in the stone canal interradius. 



