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 "hydrocoele 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. Echinus 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 (Theel 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 In Toxopneustes (Tennent, unpublished 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 hydroccele 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 



