4 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



expect their development to afford features of unusual interest. However, 

 the fact that Ptilometra mulleri carries its Pentacrinoids on the cirri = proves 

 that at least this species has no prolonged free-swimming larval stage. I have 

 not sufficient material of Thalassometrids at my disposal for investigating 

 the character of their eggs; only in Parametra orion (A. H. Clark) I find the 

 eggs to be a little smaller than those of Tropiometra, namely, 0.15 mm. 

 against about 0.2 mm. in Tropiometra. 



Now, as regards Tropiometra, the larvae are, as ah-eady stated, not plutei 

 or anything like that. Still there is something in the development to account 

 for the wide distribution of this species. First the egg is free, probably 

 pelagic, and then the larvae, which are very active swimmers (in the jars they 

 generally were found swimming at the siu-face), may keep up their pelagic 

 existence for quite a long while. If they find a suitable place for fixation 

 they may attach themselves when only 2 or 3 days old; otherwise they may 

 swim for 6 or 7 days, and I had one specimen which did not attach itself 

 until it was 8 days old. Due to this facultative, prolonged swimming period 

 the larvae may be carried for considerable distances by currents, and this 

 accounts in a natural way for the ^\ade distribution of the species. 



An interesting fact to notice in this connection is that none of the speci- 

 mens from Tobago proved to be infested with Myzostoma. This would seem 

 to indicate that the colony hving here must be an isolated one, originating 

 from larvse carried thither by the currents, not in direct connection with the 

 main habitat of the species in American waters, which should probably be 

 sought to the south of the Orinoco and Amazon Rivers. That specimens 

 living there are infested with Myzostoma is knovm from v. Graff, who records 

 Myzostoma gigas Liitken from specimens of "Antedon" carinata Leach taken 

 by the Challenger off Bahia.^ 



A. H. Clark (Vid. Medd., 1909, p. 184) suggests that Tropiometra carinata 

 may have extended its range northward from Brazil by passing under the 

 fresh water discharged into the sea by the Amazon and Orinoco, thus sur- 

 mounting this barrier "by the simple process of gradually increasing the 

 depth of its habitat." This explanation may account for the specimens 

 hving in deep water (200 to 300 fathoms) in the West Indian sea, but its 

 littoral occurrence in Tobago (and most probably other places in the southern 

 part of the Caribbean Sea) is more satisfactorily explained through the 

 transportation by means of currents, although there is a possibihty that the 

 httoral colony may have come from larvae of the specimens occurring in the 



' H. L. Clark (Scientific Results of the Trawling Expedition of H. M. C. S. Thetis. Mem. Austral. 

 Mus., IV, part ii, 1909) describes such specimens under the name of Himerometra pcsdophora, but A. H. 

 Clark (The Recent Crinoids of Australia, Mem. Austral. Mus., iv, part 15, 1911, p. 785) maintains that 

 they are Ptilometra mulleri. 



' L. V. Graff. Report on the Myzostomida collected during the voyage of H. M. S. Challenger. Scien- 

 tific Results of H. M. S. Challenger, vol. x, part xxvii, 1884, p. 35. Though I must doubt the correctness 

 of this identification (Myzostoma gigas is otherwise known only from the arctic Heliometra glaeialis), 

 this proves that Tropiometra carinata is the host of a Myzostoma, like other Comatulids. 



