640 Bl'LLETIX S2, UNITKD STATES NATIONAL MUSEUM VOLUME 1 



because of the reliUively great devel<)])inent of the skeletal elements, the larva must be 

 rather heavy and a poor swimmer so that it probably sinks almost directly downward 

 where it meets the tip of the upturned cirrus and attaches itself. According to Morten- 

 sen, the fact that the pcntacrinoids are not found attached to all the upturned cirri 

 but generally to only 2 or 3 of them would seem to indicate a slight amount of swun- 

 ming, as otherwise one would expect tliem to be attached to the cirrus just below the 

 point of origin. 



Mortensen found that quite a large percentage of the pentacrinoids contain in 

 their stomachs the half-digested but still perfectly recognizable remains of larvae. 

 He even saw quite young pcntacrinoids with the vestibule just opened and the arms 

 not yet developed with an embryo almost of their own size in their mouths. He says 

 that, because of the large number of pentacrinoids found attached in clusters to the 

 tips of the upturned cirri — Andersson counted no less than 99 on one adult — this 

 danger to the embryos is very real and i)robably quite a large number thus perish. 



In the recently attached pentacrinoids the vestibule has now assumed its normal 

 position at the formerly posterior (now anterior) end, and has a distinct lumen. The 

 tentacles have not yet become free. In sections the little pentacrinoid is seen to differ 

 conspicuously from the larva in the histological character of the entoderm. A single 

 fairly regular layer of nuclei is seen along its outer surface while the whole inner part is 

 filled with a dense mass of small granules that stain very deeply in haematoxylin. 

 Mortensen believes that the mass occupying the entoderm consists of wandering cells 

 derived from the mesoderm which take an active part in the histolysis of the entoderm 

 that occurs during the metamorphosis. 



With the removal of the vestibule from the ventral side of the embryo to the 

 posterior end, the hydrocoele and the coelomic cavities have also come to occupy their 

 definitive positions. The inner wall of the vestibule is very thick, and between the 

 nuclei there are many of the granules just mentioned, scattered or in dense groups. A 

 slight concavity in the middle of the floor of the vestibule is the first indication of the 

 future esophagus. Below this, the ectoderm and entoderm are fusing in the center of 

 the still open hydrocoele ring. 



The stone canal has not yet opened into the parietal canal, and the pore canal is 

 closed off from the exterior. In the mesentery within the parietal canal there is a slight 

 accumulation of nuclei which probably represents Russo's "primary gonad." 



In a somewhat more advanced stage the tentacles have protruded into the vesti- 

 bide, which is about to open, a depression having appeared in the middle of its outer 

 wall and the 5 oral valves being about to separate. The mouth and esophagus have 

 been formed, and in the stomach a lumen is beginning to appear, the granular mass 

 being about to be absorbed. The intestine now has been differentiated, but the anal 

 opening has not as yet been formed. The stone canal has opened into the parietal 

 canal, but no external opening of the pore can be discerned. The chambered organ is 

 beginning to assume its typical form and the axial organ is distinct. It appears as if 

 the parietal canal had opened into the oral coelome, but Mortensen coidd not definitely 

 determine this. 



The next important change is the opening of the vestibide. The young 

 pentacrinoid now begins to feed directly, having until now subsisted on the yolk con- 

 tained in the egg. Diatoms, chiefly of the Coscinodiscus type, arc found in the stom- 

 ach, and in some cases also larvae. 



