60 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



and metamorphose into the Pentacrinoid. That this is a general rule among 

 the Comatulids may be concluded from the fact that in so many cases the 

 Pentacrinoids are found attached to the cirri of the mother specimen, which 

 would not be the case were the eggs free. The only exceptions from this rule, 

 so far as hitherto known, are Tropiomdra on one side and the 3 viviparous 

 forms on the other side. In Tropiometra the eggs are not retained on the pin- 

 nules, but either drop to the bottom or, what is more probable, float in the 

 water. The special development of the egg-membrane into a densely spinous 

 structure, recalling what is found in some pelagic fish eggs, as those of Calli- 

 onymus, decidedly points in the direction of the eggs being pelagic. 



Whether pelagic or, in any case, free eggs are to be found in other Coma- 

 tulids is hard to predict. 37 One might be inclined to expect them in forms with 

 small eggs, as are found, according to A. H. Clark, especially in the Thalasso- 

 metrids ; but then the difference in the size of the eggs of Comatulids does not 

 appear to be in any way conspicuous. Thus in Antedon adriatica the eggs are 

 (Seeliger, p. 173) about 0.25 mm. or almost exactly the same size as those of 

 Tropiometra, the former having them retained on the pinnulse, the latter 

 having them free. This fact would seem to indicate that the size of the eggs 

 is of no great value in this regard. More importance may be attached to the 

 geographical distribution, free pelagic eggs and larvae being more likely found 

 in species with a wide distribution than in those having a more restricted 

 occurrence. It was, in fact, the wide distribution of Tropiometra carinata 

 that led A. H. Clark to suggest that this species might possibly have a true 

 pelagic larva (see page 3). Although this suggestion did not hold good, 

 the reasoning was correct; the developmental history of this species did give 

 the clue to its exceptional distribution. In the same way, Clark is probably 

 right in concluding, from the markedly solitary habit of the Thalassometrids, 

 combined with the fact that the egg is somewhat smaller than usual in Coma- 

 tulids, that these forms must have a prolonged pelagic stage, 38 either the eggs 

 being pelagic or only the larvae having a prolonged free-swimming period. 



Of special interest is the discovery of two new cases of care of the brood in 

 the Comatulids Notocrinus virilis and Thaumatomctra nutrix, only one sin- 

 gle case, Isometra vivipara, being known previously. 39 The latter evidently 

 represents the least specialized case. Here the larvae still retain their typical 



37 Quite recently, also, Antedon petnsus (Diiben and Koren) has been found to have free eggs like Tropio- 

 metra (Th. Mortensen: Notes on the development and the larval forms of some Scandinavian Echino- 

 derms. Vid. Medd., vol. 71, 1920, p. 150). 



38 A. H. Clark. On a collection of Crinoids from the Zoological Museum of Copenhagen, Vid. Medd., 

 1909, p. 122. 



39 Possibly tlimcromctra pvdophora H. L. Clark (according to A. H. Clark Ptilomelra mullcri A. H. Clark) 

 also protects its brood. H. L. Clark has found a number of young Pentacrinoids attached to the pinnules 

 of the species. (Scientif. Res. of the trawling expedition of H. M. C. S. Thclis, Echinodermata. Mem. 

 Austral., Mus., iv, 1909, p. 525.) But he does not give any more detailed information of the way in which 

 they are attached or whether the eggs are kept in the usual way, in clusters round the genital openings or in 

 a special marsupium. Judging from his figure of the youngest Pentacrinoid (plate XLVII, figure 7) it would 

 appear that they are attached to the dorsal side of the pinnules, so that this case in no way corresponds to 

 what obtains in Thaumatometra nutrix. 



