302 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



is the extra ray, as six ambulacral grooves, all, so far as can be seen, exactly alike, 

 converge at the mouth. An examination of the radials and postradial series furnishes 

 no clue, as they are all quite alike. One specimen has the additional ray interpolated 

 between the two on the left side, one has it behind the right posterior ray, and 7 have 

 it inserted behind the left posterior ray (see Part 2, figs. 729-733, p. 346). 



In a specimen in the Museum of Comparative Zoology one of the arms divides on 

 the thirty-ninth brachial (see Part 2, fig. 173, p. 89). 



A specimen from Rio de Janeiro in the Berlin Museum has 11 arms, one IIBr 2 

 series being present. 



Dr. H. L. Clark said that at Tobago no individuals with more or fewer than ten 

 arms were noted in more than 200 examined, but several cases of arms forked distally 

 were seen, and in one case a forked pinnule was noted. As a rule the arms were ap- 

 proximately equal, but in some individuals those of one side were distinctly shorter 

 than the others. In such cases, however, it was usually obvious that the short arms 

 were regenerating. 



Early stages. The early stages of this species have been described by Dr. Th. 

 Mortensen from material coUected at Tobago, where it occurs in fair numbers in 

 places on the coral reef at the western end of the island, sometimes in such shallow 

 water that it is exposed at the lowest tide. Mortensen says that this is a very hardy 

 species, and there was no difficulty in keeping specimens alive in jars, even for several 

 days. Late March and April were just in the breeding season, and the first lot of 

 specimens gave a few larvae. After this he succeeded repeatedly in getting cultures 

 of larvae, never in great numbers, but sufficient to enable him to secure material for 

 a fairly complete study of its development. 



As the red-brown larvae are quite opaque, nothing of the interior structure and 

 its successive transformations could be seen on the living object. Only the first 

 cleavage stages could be studied directly; from the gastrula onward all the develop- 

 mental processes, excepting the development of the skeleton, must be studied by 

 means of sections. 



The relative scarcity of the material, together with the not very good preserva- 

 tion and the difficulty of the orientation of the embryos in the younger stages pre- 

 vented Mortensen from making a complete study of every detail in the developmental 

 processes. Also the very rapid succession of the different stages in the development 

 added considerably to the difficulties in securing every stage. 



The egg is free, probably pelagic, and the larvae, which are very active swimmers 

 and in the jars were generally found swimming at the surface, may keep up their 

 pelagic existance for quite a long while. 



If the larvae find a suitable place for fixation they may attach themselves when 

 only two or three days old; otherwise they may swim for six or seven days, and Morten- 

 sen had one specimen that did not attach itself until it was eight days old. Mortensen 

 says that as a result of this facultative prolonged swimming period the larvae may be 

 carried for considerable distances by currents, and this accounts in a natural way for 

 the wide distribution of the species. 



In accordance with the comparatively long period of active swimming of the larvae 

 the pentacrinoids are not found attached to the cirri or other structures of the adults. 

 Indeed, he found no free pentacrinoids. Not knowing whether the larvae have any 



