140 Papers from the Department of Marine Biology. 
DISCUSSION. 
The development of Laganum, while conforming essentially with the 
general method of growth of other echinoderms, possesses many inter- 
esting differences as to details, as may be seen even in an investigation 
necessarily as incomplete as is afforded by a study of one period of its 
life-history. 
RATE OF GROWTH 
One of the most unusual features of the pluteus under considera- 
tion is its rapid rate of development as compared with that of other 
described forms. In Echinocardium cordatum (MacBride 4) the first 
trace of the ‘‘Echinus rudiment’’—that is, the lobe of the enteroccele 
representing the rudiment of the water-vascular system and the invagi- 
nation of the ectoderm subsequently forming the amniotic cavity— 
occurs when the animal is 9 days old. By the tenth to the twelfth 
day the ‘‘hydroccele has become marked into incipient lobes which are 
the rudiments of the radial water-vascular canals and of the primary 
tube-feet of the adult,” and at 18 to 22 days the formation of adult 
spines commences. Lchinus esculentus (MacBride 3) requires norm- 
ally 16 to 17 days to attain a stage of development corresponding to 
that of Echinocardium cordatum at 9, while the spines of the adult do 
not appear until the thirty-third to the thirty-sixth day. In Echino- 
cyamus pucillus (Théel 7) the primary tentacles are well formed at 
12 days, and shortly afterward the calcareous plates and skeletons of 
the spines are laid down. In Toxopneustes (Tennent, unpublished 
notes) while the early development is comparatively rapid, the ‘‘ Echi- 
noid rudiment” is not well formed until about the twenty-fifth day. 
In Laganum, at 29 hours, the hydroceele ring is already closing and its 
lobes have differentiated into the primary tentacles. The amniotic 
cavity occupies a large part of the pluteus and is crowded with well- 
developed spines. At 55 hours the entire oral surface is covered with 
permanent plates bearing the skeleton of the spines, while the larval 
skeleton has attained a stage of wonderful complexity. Other forms of 
approximately the same age are in the early stages of larval devel- 
opment. Echinocardium cordatum (MacBride 4) at 30 hours has 
merely completed gastrulation, while Echinus esculentus (MacBride 3) 
at 1 day has just become a fully formed and free-swimming blastula. 
Toxopneustes (Tennent 6) reaches the pluteus stage at 24 hours. 
FORMATION OF THE AMNIOTIC CAVITY. 
The amniotic cavity, as stated above, is already present in the 
earliest stage of my material, so that necessarily no positive statement 
as to its method of formation can be made. Although the possibility of 
a lateral invagination and a secondary shifting is not absolutely ex- 
cluded, the unusual position it occupies and the position of its external 
opening permit the supposition that it may have developed in a manner 
