394 DR ARTHUR T. MASTERMAN ON THE 



Asterina and Antedon, that it must be regarded as the normal mode of development 

 for Echinodermata. The development here described must therefore be considered as a 

 remarkable exception. There is every reason to suppose that a similar mode of develop- 

 ment holds in the case of Solaster, and also in Ophiura brevispina. In the latter 

 species, Grave* has described certain stages of the larval development. The larva is of 

 a free-swimming, demersal type, in some respects resembling that of Antedon. His 

 first note was somewhat unintelligible from lack of data, and his fuller paper* does not 

 clear up the main points. He naively states that all his earlier stages were destroyed, 

 as he did not know that any special interest would be found, and he commences with 

 the early larval stage, corresponding to the last embryonic stage of Cribrella. This 

 larva exhibits an archenteron filled with hypenchyme, and its appearance leads the 

 author to conjecture that invagination does not take place, and that " the larva before 

 gastrulation is a solid, planula-like affair, and later the archenteron is formed by a 

 splitting away of the central core. In the same way the plug of cells is probably 

 formed by the hollowing out of the solid archenteron " (pp. 87, 88). In the light of 

 the singular resemblance indicated, it is the more regrettable that Grave did not 

 examine the early stages, for the resemblance serves to cast grave doubt on the 

 ' probabilities ' suggested. 



A strict regularity of segmentation appears to hold principally in the Holothuroidea 

 (Selenka t and ClarkJ) producing an equal segmentation. On the other hand, certain 

 echinids, e.g. Strongylocentrotus lividus (Selenka) and Echinocardium cordatum 

 (Fleischmann §) appear to have a more unequal segmentation after the division into 

 four blastomeres. At the same time a hollow blastula is produced, although the cells 

 are unequal at the two poles. Slight inequalities have also been observed in Asteroidea 

 (Lxjdwig||), but these are apparently the nearest approaches to the condition in Cribrella. 

 The unequal segmentation, followed by the production of a solid morula, appears, with 

 our present knowledge, to be unique for echinoderms. The same remark applies to the 

 process of multiple egression and invagination, with the formation of hypenchyme. 

 For somewhat similar types of early development we have to go to the Alcyonaria and 

 Zoaniharia. In each of these groups a solid morula is very common, and in numerous 

 instances the later larva is described as having an archenteron, filled with a mass of 

 cytoplasm and nuclei of a hypenchymatous character. It appears to have been usual 

 to conclude that the solid morula was converted by delamination into the solid planula, 

 but JourdanH has noted that in Actinia equina there is complete invagination, followed 

 by a filling up of the archenteron by a mass of yolk granules, the origin of which is 

 doubtful; and Kowalewski ## describes in a species of Astraea a larva whose archenteron 



* Grave, Caswell, Johns Hopkins Univ. Memoirs, iv. 

 t Selenka, E., Zeitschr. tviss. Zool., Bd. xxvii. 

 X Clark, H. L., Johns Hopkins Univ. Memoirs, iv. 

 § Fleischmann, A., Zeitschr. wiss. Zool., Bd. xlvi., 1888. 



|| Ludwig, H., " Entwicklungsgeschichte der Asterina gibbosa," Zeitsch. w. Zoo!., vol. xxxvii., 1882. 

 1" Jourdan, E., Ann. Sci, Nat. Series, 6, T. x., 1879-80. 

 ** Kowalewski, A., Jahresb. Anat. u. Physiol., 1873. 



