NOTOCRINUS vmiLis. 53 



mother animal, which does not appear to be the case in Isometra vivipara, 

 where the larvse remain within their egg- membrane all the time till they leave 

 the marsupium, and do not reach any unusual size. 



The mesenchyme is very extensively developed, as might be expected 

 from the large size of the larva. It has a distinctly fibrillar structure. In 

 the anterior part (plate xxv, figures 1 to 3) one can distinguish an outer 

 layer with very few nuclei and much developed fibrillse from an inner part, 

 with numerous nuclei and fibrillae only slightly developed. There may be a 

 fairly distinct hmit between the two parts. In the inner part (which sur- 

 rounds the chambered organ and the stalk-joints) are found, besides the 

 nuclei, a varying number of yolk globules, 

 single or connected into small ball-like 

 masses (plate xxvi, figure 8). Sometimes 

 such yolk globules may be found also in the 

 posterior end of the larva, but they are much 

 less numerous here than in the anterior end. 



The skeleton is very nearly in the same 

 stage of development in all the larvse, in spite 

 of the considerable variation in their size. As F'"- v -Primary and supplementary 



rv 1 r. 1 termmal stalk-plates of Nolocnnus nr- 



represented in plate xxiv, figures 2 and 3, the ms. xieo. 

 circles of oral and basal plates are typically 



developed, and there are 4 good-sized infrabasalia. In one case I can distin- 

 guish 5 infrabasalia, the two being much smaller than the three others. There 

 are about 25 stalk-joints and a terminal stalk-plate of usual size. But here 

 again we meet \vith a novel feature: Beside the large terminal plate there are 

 some smaller supplementary plates, varying in number from 1 to 5 (text- 

 figure 7). They he without any definite order around the border of the pri- 

 mary terminal plate. In a few specimens (in which the skeleton is in a slightly 

 younger stage of development than in the specimen represented in plate 

 XXIV, figure 3) there are as yet no supplementary terminal plates; but there 

 is no reason to suppose that they would not have appeared here also in 

 due time. 



The meaning of these supplementary terminal plates is hard to guess. 

 It may perhaps be supposed that there is something in the mode of attach- 

 ment of the Pentacrinoid that has caused their appearance. It is a pity 

 that only this one stage of the development of this unusually interesting 

 Crinoid should be available. Owing to the lack of ciliated bands the larva 

 must be unable to swim and must simply drop to the bottom on leaving the 

 marsupium, or it may attach itself to the walls of the marsupium, the head 

 of the Pentacrinoid protruding through the marsupial opening, as is the case 

 in Thaumatometra nutrix. But as the Pentacrinoid of the latter species has 

 no terminal plate, it is hard to see why the Pentacrinoid of Notocrinus should 

 have an extra number of terminal plates. Only direct observations can give 

 the answer to these interesting problems. 



