542 INVERTEBRATA CHAP. 



twelve days, but an Auricularia larval stage is never developed, and 

 the larva in its earty stages takes no food. The mesenchyme 

 appears to be formed in the blastula stage, as in Echiuoidea and 

 Ophiuroidea, from one side of the blastula wall; it is this area 

 which is later invaginated to form the archenteron. The coelom is 

 separated off and divides exactly as it does in Holothuria and 

 Synapta. Then the stoinodaeum is formed, and the cilia are reduced 

 so as to form five transverse rings. Were it not that a prae-oral 

 region is retained which is covered with cilia, also an anal field in 

 the neighbourhood of the anus, we might compare the larva to the 

 pupa of Synapta. 



Ludwig was able to determine that the covering of the buccal 

 tentacles, and also the nerve-ring which connected their bases, took 

 their origin from the ectoderm lining the larval stouiodaeum. The 

 tentacles arise from the radial canals and protrude into an atrium 

 which is the larval stoniodaeum. These radial canals are the first 

 and only lobes developed from the hydrocoele ring. As the prae-oral 

 lobe diminishes the mouth is gradually shifted to the anterior pole, and 

 the cilia disappear. The paired tube feet appear on the radial canals, 

 first on the mid-ventral one, and then, much later, on the lateral 

 canals, and lastly on the dorsal one (Fig. 402). Unpaired tube feet 

 seem not to be developed, i.e. the tips of the radial canals do not 

 protrude as azygous tentacles. The ectoderm covering the tube 

 feet is developed as nervous discs before the outgrowths from the 

 radial canals appear. As soon as the first tube feet have developed 

 the young Cucumaria sinks to the bottom and begins its creeping life. 



An examination of young imago-stages of Cucumaria, just after 

 the metamorphosis is complete, yields many most interesting results. 

 Ludwig was able to keep the young Cucumaria planci alive for 

 four months, and we ourselves have examined a series of young 

 Cucumaria from the Antarctic (1912). The pore-canal is lost, but 

 there is an anterior coelom embedded in the body-wall in which the 

 stone-canal ends. This anterior coelom was seen by Ludwig, who, 

 however, regarded it as a secondary evagination of the stone-canal. 

 The walls of the atrium, i.e. the larval stouiodaeum, have split into 

 five valves (v, Fig. 402), and each valve is supported by a calcareous 

 plate. A perfect cuirass of overlapping calcareous plates is found 

 everywhere in the skin. The alimentary canal is still straight, 

 but as the animal grows older the intestine lengthens and becomes 

 thrown into a forwardly directed loop, and ciliated pores appear 

 connecting the anterior coelom with the body-cavity, and in this way 

 the secondary or internal madreporite is formed. 



On reviewing the comparatively disconnected series of facts 

 which we have just considered, it is clear that the larva of 

 Holothuroidea, in spite of its external similarity to the Bipinnaria, is 

 profoundly dissimilar when its internal anatomy conies to be con- 

 sidered. Thus, though both types of larvae possess a prae-oral lobe, 

 that of Bipiunaria contains extensions of the left and right coelomic 



