52 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



xxiv, figure 9; plate xxvi, figure 4). The parietal canal has a long, narrow 

 anterior prolongation (plate xxiv, figure 4; plate xxv, figures 1 to 3). 



The coelomic vesicles have assumed their normal position, the left at the 

 posterior, the future oral end, the right at the dorsal side. The former 

 prolongs upwards on either side of the hydrocoel (plate xxv, figure 6). The 

 chambered organ has been formed and may be followed to near the anterior 

 end (plate xxv, figures 1 to 3). 



While there is nothing unusual in the relations of the entoderm, the hydro- 

 coel, and the parietal canal, a very unusual feature is apparently connected 

 with the oral coelom. In the posterior end of the embryo a number of fairly 

 conspicuous sacs are seen, looking like large glands, apparently opening out- 

 wards (plate xxiv, figure 2). As seen in transverse sections (plate xxvi, 

 figure 3), they may be fairly regularly arranged, three or four of them on 

 each side; in other cases they are found only on one side, as seen in the frontal 

 section (plate xxiv, figure 7). I think it fairly safe to conclude that these 

 sacs originate as prominences from the posterior wall of the (later) oral 

 ccelom (plate xxv, figure 8). They are afterwards completely separated off 

 from the coslomic wall and acquire an outward opening through the body- 

 wall (plate xxvi, figure 5). The two figures quoted would seem to leave no 

 doubt that such is their history, but it is impossible to follow the whole 

 process on the single larval stage available. However, it should be recalled 

 that the specimen from which plate xxv, figure 8, was drawn is exceptional, 

 as mentioned above (p. 49) in having the vestibulary invagination in a 

 much younger stage of development than the other larvae, which seems to 

 accord well with the fact that also the gland ular sacs are in a younger stage 

 of development in this specimen, while none of the other larvae showed this 

 young stage in the development of the sacs. As regards the outer opening 

 shown in plate xxvi, figure 5, it must be admitted that it is not as distinct 

 as here represented, the opening itself being not quite clear. But I have no 

 doubt that my interpretation is correct. 



The histological structure could be made out remarkably well (plate 

 xxv, figure 7; plate xxvi, figure 11). They consist of a single layer of cells, 

 with nuclei somewhat larger than those of the surrounding mesenchyme. 

 On the inner end of each cell is seen a mass of small globules, which stain 

 yellow with picrocarmine and appear conspicuous against the red nuclei 

 and the finely granulated cell-protoplasm. As appears from plate xxvi, 

 figure 5, this structure disappears towards the opening of the sac. It is 

 evident that these sacs must be glandular in nature, the like of which is 

 otherwise unknown in Crinoid larvae, unicellular glands being the rule such 

 as are also found in this larva, as stated above. It is quite a novel structure, 

 which one might well suggest may have some relation to the life-conditions 

 of this larva in the marsupium of the mother animal, conditions which are 

 apparently very different from those of the larvae of Isometra vivipara, so 

 far as they must be supposed to derive nourishment in some way from the 



