264 PHYLUM ECHINODERMATA. 



Gunn.) but the Synaptidae may attain a length of from one to 

 two metres. 



Certain species of the genera Holothuria and Stichopus form 

 an important article of commerce in the East, being used by 

 the Chinese for the preparation of a highly esteemed soup, etc. 

 They are known as Trepang and Beche de mer. 



They are not suitable for preservation as fossils, but remains 

 of their spicules have been described from the Carboniferous, 

 the Jurassic, the Cretaceous and the Eocene formations (Synapta. 

 Chiridota, Myriotrochus). 



The development is sometimes direct, but more usually there 

 appears to be a bilateral larva called Auricularia which arrives 

 at the adult condition by passing through a barrel-shaped 

 pupa stage. In Phyllophorus urna Gr., Synapta vivipara 

 Oerst., and Chiridota rotifer a Pourt., the eggs make their way 

 into the body-cavity, where they are fertilized and undergo their 

 development. This phenomenon has been examined in the 

 case of Synapta vivipara by Clark * who states that the eggs 

 escape into the body-cavity through the walls of the genital 

 tubes and that the spermatozoa make their way into the sea 

 through the genital duct and then enter the cloaca, through the 

 walls of which, either by means of fine pores or by actual pene- 

 tration, they enter the body-cavity and fertilize the ova. The 

 young escape from the body-cavity by rupture of the body 

 wall or of the intestinal wall. In some forms the eggs are 

 received into two ventrally placed pouches (Cucumaria minuta 

 Fabr. (glacialis) and laevigata Verr.), in others into a dorsal 

 pouch (Psolus ephippifer W. Thorns.). In some they are 

 attached to the dorsal integument (Cucumaria crocea Less.), 

 and in Cucumaria planci the eggs are retained for some 

 time amongst the tentacles. As a rule, however, both ova 

 and spermatozoa are spawned direct into the sea, and 

 the young are developed independently of the mother. Direct 

 development within the egg membranes occurs in Cucu- 

 maria Jcirchsbergii (Psolinus brevis, Kowalevsky, op. cit.), and 

 possibly in those forms which develop in brood-pouches. In 

 Phyllophorus urna (Kowalevsky, op. cit. ) a ciliated larva is formed, 

 which swims about in the body-cavity and possesses 5 tentacles 

 and 2 tube-feet when it leaves the parent. In Cucumaria planci 

 * Mem. Boston Soc. Nat. Hist., 5, 1898, p. 53. 



