EARLY DEVELOPMENT OF ORIBRELLA OCULATA, 383 



the egg, is of a uniform warm orange tint, and has this appearance throughout its larval 

 life. 



Stage B — (figs. 21 and 22). — This stage is reached by a somewhat rapid elongation 

 of the anterior part of the larva, causing a marked difference in its general appearance. 

 There is now no difficulty in recognising its points of symmetry ; and as it also becomes 

 somewhat flattened from side to side (fig. 22), it always rests on one side or the 

 other after death. In life it has similar habits to stage A. In some specimens there is 

 still a trace of the blastoporic pit, but in most there is none. 



Stage C — (figs. 23, 24 and 25). — Little increase in size has been effected by this stage, 

 but the anterior end has now become boldly indicated by the presence of a median 

 dorsal process at the anterior end of the larva, quickly followed by the protrusion of 

 a median ventral process at its base. These processes are evidently parts of the pre- 

 oral lobe, and together constitute a homologue of the so-called ' larval organ ' of the 

 Asterina larva. This term appears to be objectionable, as tending to emphasise an un- 

 natural distinction between the ' larval ' anterior part and the ' adult ' posterior. 

 Nothing is more clearly indicated by recent echinoderm research than the fact that the 

 whole organism is a larva at its early stages, becoming almost in its entirety the adult 

 organism, a portion of the pre-oral lobe alone being lost. The conception of an adult 

 growing upon a fostering larval portion and eventually metamorphosing is not in 

 accordance with the facts, at least of asterid development. A clear recognition of these 

 facts will focus the questions of orientation to which reference will be made later. 



Stage D — (figs. 46, 47, 48, 49). — In this stage the ventral pre-oral process divides 

 into two. These two form with the dorsal process a tripod, upon which the larva rests 

 after fixation (fig. 48). These three processes, a further development of the ' larval organ ' 

 in Asterina (Ludwig), are apparently homologous with the three brachiolar arms of 

 the Brachiolaria. Lcjdwig found that some specimens of Asterina had the ventral 

 part of the larval organ partially forked, producing an even closer resemblance to the 

 three processes in Cribrella. MacBride does not agree with Ludwig's comparison, and 

 regards the adhesive disc as being the homologue of the larval organ. 



The outline of the larva is now more defined, with a slight constriction or ' neck ' 

 behind the pre-oral processes. The cilia on these processes appear longer and more 

 numerous than those on the posterior part of the body. At this stage the larva is still 

 perfectly piano-symmetric. It swims actively, head first, and with its ventral surface 

 downwards. Larvae at this stage may be readily kept in a small vessel for several 

 days ; they travel about very slowly and methodically, just clear of the bottom, and 

 very commonly fix themselves temporarily to the side of the vessel, occasionally to the 

 bottom ; in. several cases they also lie on their right side for a short time preparatory 

 to renewing their movements. It is a little difficult to reconcile these active and 

 fixative habits with their natural surroundings. It is quite common to find mothers 

 with a brood of young starfishes as advanced as stage F, yet the brood-cavity is too 

 confined for the execution of any movements at all comparable to those performed by 



