634 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME I 



The first three pinnules are fairly robust; P t is from 5 mm, to 10.5 mm. in length 

 and composed of 10 to 17, usually 13 to 14, segments, which are short and smooth. 

 P! is always longer and usually stouter than P 2 and P 3 . P 2 is of 9 to 14, usually 10 to 12 

 segments and 4 to 8.5 mm. long. P 3 is of 9 to 14, usually 10 to 12 segments and 4 to 

 7 mm. long. P 3 generally has the ambulacral groove developed. 



The first genital pinnule is P 4 , P 5 , or P 6 , usually P 5 . The last may be as far out as 

 P 28 or even further, so that only some 6 or 7 pinnules at the arm tips are devoid of 

 genital organs. Often pinnules without the usual expansion occur among the genital 

 pinnules. According to Mortensen these are regenerated pinnules, as may be inferred 

 from their occurrence at different stages of growth. The very characteristic appearance 

 of the female genital pinnules is due to the enormous widening of the third and fourth 

 segments, forming a brood chamber; this widening is mainly on the aboral side of the 

 segments. In the lower part of the arms the fifth segment is also slightly broadened. 

 Further out on the arm the broadening diminishes, first the fifth then the fourth 

 segment assuming the normal appearance, the broadening of the third alone remaining. 

 A corresponding development of the genital pinnules takes place with the growth of 

 the individual, the young having only the third segment on the genital pinnules of the 

 lower part of the arm broadened, the more distal pinnules showing no broadening at all. 



In the males the genital pinnules are distinctly broadened, though this is not so 

 extensive as in the female. It begins on the third segment, which is the widest, and 

 continues, gradually diminishing, to about the middle of the pinnule, being almost 

 equally developed on both sides of the segments. The distal segments of the genital 

 pinnules are fairly slender and smooth, the distal ends of the outer ones being only 

 slightly spiny. The articulations are not swollen. 



The distal pinnule ambulacra have side and cover plates of which the former are 

 more or less rodlike, sometimes branching at the outer end and the latter are irregularly 

 fenestrated; both series may be reduced or even absent. The tentacles usually have 

 some thorny rods but these too may be absent. 



Sacculi are in general fairly regularly developed on the pinnules, less so on the 

 arms; they appear to be entirely wanting along the ambulacral furrows on the disk. 

 They are very pale and inconspicuous in the preserved specimens. 



The disk is naked. The mouth is slightly excentric. The anal cone is near the 

 oral angle of the posterior interradius. The interradial areas may be quite narrow, 

 pressed inward by the cirri rising between the arms. 



Abnormal specimens. In one instance P< is double, a small additional pinnule 

 being developed at its base but independent of it, and running downward along the 

 arm. This supplementary pinnule possesses an ambulacral groove. 



Mortensen also found a pinnule which bifurcated on the fourth segment. 



Variations [by A.M.C.]. Since I. vivipara is one of the few species of the genus 

 known from more than a few specimens, I give here a table enumerating some measure- 

 ments of specimens in the British Museum as well as of two of the syntypes. I am 

 indebted to Mr. P. A. Andersson of the Stockholm Museum, who sent me most of the 

 type material. Unfortunately, only the specimen from Swedish South Polar Expedition 

 station 58 and four of those from station 59 have retained their station numbers, botli 

 of these being on the Burdwood Bank. The other stations from which the types came 

 range from off southern Brazil to the colder water of the Weddell Sea but, nevertheless, 

 all the remaining specimens appear to be of the same species. Thanks to Dr. F. 



