ISOMETRINAE 187 



The species protects the brood Hke /. vivipara and I.flavescens and, as in those species, 

 the third and fourth segments of the genital pinnules of the females are enormously 

 expanded, mainly on the aboral side, to cover the ovary and brood-pouch. In the middle 

 genital pinnules the fifth segment is expanded to a smaller degree. 



The third and succeeding segments of the male genital pinnules are expanded to 

 cover the testes. The expansion is a little stronger on the aboral than the oral side, but 

 it is not so asymmetrical as, and it is much smaller than, in the female ; the expanded 

 segments gradually merge into the unexpanded distal segments. 



The contents of one brood-pouch of the female from the Palmer Archipelago were 

 examined. There is one egg, one larva similar to that of/, vivipara figured by Mortensen 

 as a full-grown larva (1920, pi. xxii, fig. 8), and three other larvae with the plates better 

 developed so that the orals and basals touch one another — a stage intermediate between 

 Mortensen 's figures 8 and 9 {ibid.). They are of the same size as those of /. vivipara. 

 Similar larvae occur in the brood-pouches of the female from the Ross Sea ; in one 

 brood-pouch there were seven. Those of the specimen from St. 1872 are younger. 



On the disks of the Palmer Archipelago specimens there is at the apex of each inter- 

 radial area a sharp calcareous plate with its apex projecting over the peristome' 

 (Fig. 17 c). A broad depression runs radially along each plate because its sides are 

 curled upwards. The bases of the plates are not distinct but they appear to be straight. 

 Similar plates are present in one of the Ross Sea specimens and appear to be present 

 in two others ; in the fourth, and in the specimen from St. 1872, the disk cannot be seen. 

 Clark (1915 b, pp. 340-1) says that the oral plates of young comatulids "are always re- 

 sorbed long before aduh life is reached, no trace of them whatever remaining"; these 

 plates of Isometra graminea must therefore be, what Clark calls in the same place, 

 secondary perisomic orals, though he describes them as occurring only in certain species 

 in which the disk is heavily plated; there are no other plates on the disk of /. graminea. 



The sacculi are inconspicuous. They are often fairly regular on the pinnules, less so 

 on the arms ; they occur on the disk. 



The pinnule ambulacra are protected by large side- and cover-plates (Fig. 17 d), 

 three pairs to each segment. The side-plates overlap one another so as to make a con- 

 tinuous wall: the divisions between them are difficult to see. The cover-plates are more 

 rounded with a fan-like system of supporting rods terminating in peripheral spikes ; in 

 this they somewhat resemble the cover-plates of /. lineata, I. angustipinna, I. vivipara 

 and /. flavescens. The tentacles contain strongly knobbed spicules. 



The species is altogether less robust than /. vivipara and /. flavescens, and may easily 

 be distinguished from them by the diff"erences in the proportionate sizes of the oral 

 pinnules. It is as readily distinguished from the much more robust /. hordea by the 

 differences in the cirri. 



1 All five plates are easily seen in the broken specimen. Two can be seen in the female and one in the 

 male between the more widely separated arms : I have no doubt there are five present. 



9-2 



