xvi ECHINODERMATA 541 



Holotlmroids as in Asteroids, Ophiuroids, and Echinoids. The 

 Pentacula stage is soon passed, since accessory tentacles arise just 

 over the spots where the radial canals are formed in Synaptct digitata, 

 and, in S. vivipara, from the rudiments of these canals. Thus we 

 have a ten-tentacled stage, and the full adult condition is attained 

 by the development of accessory tentacles in the dorsal interradii. 



OTHER HOLOTHUROIDS 



The only other of the species whose development exhibits an 

 Auricularia larva, about whose life -history anything is known, is 

 Hblothuria tubulosa. For this we are indebted to Seleuka (1876), 

 who secured great numbers of adult individuals, and had the luck 

 to obtain a ripe male and female which spawned spontaneously, and 

 in this way a natural fertilization was obtained. The egg segments 

 with great regularity, and gives rise to a spherical blastula of about 

 200 cells, which acquire cilia ; the embryo then escapes from the 

 egg-membrane and becomes a larva in the blastula stage, as in 

 Asteroidea, Ophiuroidea, and Echinoidea not, as in Synapta, in the 

 gastrula stage. Then one side of the blastula becomes thicker than 

 the rest, and becomes flattened. On this flattened surface the 

 archenteric invagiuatiou appears, and from its apex, even when 

 slightly developed, copious mesenchyme cells are given off. 



The archenterou only projects a short distance into the blastocoele, 

 so that there is a large prae-oral section of the larva. From the 

 apex of the archenteron a large coeloinic vesicle is given off, and the 

 longitudinal ciliated band becomes established in the usual way, and 

 then, afterwards, a vertical outgrowth is developed which forms the 

 pore-canal ; so that in this respect also Holothuria agrees with other 

 Echinoderms and differs from Synapta, in which the pore-canal 

 and madreporic pore are formed before the coelom has separated 

 from the archenteron. The stomodaeum appears on the ventral 

 surface as usual, and so the gut is completed, but the intestine 

 bends forward as it does in other Echiuoderm larvae. The 

 coelomic vesicle divides into anterior and posterior portions, and 

 the latter divides into right and left halves, just as in Synapta. 

 The hydrocoele develops five lobes, but beyond this stage Selenka 

 could not rear the larvae a fact much to be regretted, as the 

 development, so far as it goes, shows signs of conforming in every 

 way more closely to the type exhibited by other Echiiioderm larvae 

 than does that of Synapta. 



The development of Cucumaria planci has also been worked out, 

 so far as external features are concerned, by Selenka (1876), and his 

 account, in which there are many errors, has been corrected by 

 Ludwig (1891). The eggs were obtained by spontaneous spawning, 

 and a natural fertilization resulted. The segmentation is regular 

 and results in the formation of a regular spherical blastula, although 

 the eggs contain much yolk. Development is complete in about 



