GENERAL PART. 71 



side (plate xn, figure 2); in Isomelra a faint trace of the anterior band is 

 seen on the dorsal side. In no case have I seen the anterior band so distinct 

 as in the Mediterranean species of Antedon, as represented in Seeliger's and 

 Bury's figures. Whether the typical (or original) number is 4 or 5, is hardly 

 to be ascertained at the present stage of our knowledge of the Crinoid larvae. 



Very interesting is the indication of an extra band off the anterior end of 

 the vestibulary invagination seen in the larva of Isomctra (plate xxn, 

 figure 2), but, this is probably not a character of any greater value to the 

 morphology of the Crinoid larvse, and the same evidently applies to 

 the fact that vibratile bands are entirely absent in the larva of Notocrinus. 

 Both these cases apparently are adaptations to the special life-conditions 

 of these larvse. 



The vestibulum is always situated between the second and third band. 

 In the course of development it may either press the third band downwards, 

 as is the case in Isometra, or it may pass across the third band, which thus 

 becomes interrupted in the ventral midline, as in Compsometra and Tropio- 

 melra; also in Antedon, according to Bury and Wyville Thomson. The 

 figures given by Seeliger (taf. 16, figures 65 to 67) are rather too diagram- 

 matic to show clearly the relations to the vestibulary invagination of said 

 band, and his text (page 233) is not very clear on this point, but it seems 

 to be his meaning that the band appears interrupted but can be discerned 

 inside the invagination. This may also be the case in Tropiometra (see 

 page 11). 



Whether the relation of this third band to the vestibulary invagination 

 is of specific or generic value can not be ascertained at the present state of 

 our knowledge. At least it appears to be constant within the species. 



The shape of the vestibulum is somewhat unusual in Isometra and 

 Notocrinus, being a quite narrow slit, while in the other larvse thus far 

 known it is a broad, oval depression. This difference is evidently due to 

 the special life-conditions of these two larvae, which probably also have 

 some bearing on the peculiar fact that the lumen of the vestibulum ef the 

 Isometra larva obliterates completely at the closure, to reappear again when 

 it has assumed its final place at the posterior, or future anterior, end of the 

 metamorphosing larva. 



The suctorial disk is very indistinct in Tropiometra, a feature hard to 

 understand, as it needs means of fixation just as well as do the other Crinoid 

 larva?. On the other hand, this feeble development of the suctorial disk 

 would appear to explain the failure of so many of the Iarva3 in attaching 

 themselves (page 14) . The shape of a half -circle assumed by the suctorial 

 disk of the Notocrinus larva must evidently be a special adaptation to suit 

 the way of fixation (probably within the marsupium) adopted by this larva, 

 while the Isometra larva, which attaches itself in the usual way, has the 

 suctorial disk normally developed. 



