METAMORPHOSIS OF HOLOTHURIANS. 215 



metamorphosis being less marked i.e., growth is more 

 continuous, as in the Crinoids. 



In Holothuria tremula and Synaptula vivipara there has 

 been observed a very slight metamorphosis, the young de- 

 veloping directly in a marsupium, as in the star-fishes and 

 sea-urchins. Cladodaetyla crocea Lesson, of the Falkland 

 Islands, according to Sir Wyville-Thompson, carries its 

 young in a sort of nursery, being " closely packed in two con- 

 tinuous fringes adhering to the water-feet of the dorsal am- 

 bulacra." He also found that in Psolus ephippifer Wyville- 

 Thompson, which is covered with calcareous plates, there is 

 a dorsal group of larger tessellated plates, each supported 

 by a broad pedicel embedded in the skin. Under these 

 mushroom-like plates brood-cavities or cloister-like spaces 

 are left between the supporting columns, and in this archi- 

 tectural marsnpium the embryos directly develop into sea- 

 cucumbers. It follows that in all free-swimming Echino- 

 derm larvae, there is a true metamorphosis as distinct as in 

 the butterfly, while in other forms in which development u 

 direct the embryo is sedentary and lacks the cilia and vari- 

 ous appendages so characteristic of the ordinary larval 

 Echinoderms ; thus there are different stages in the differ- 

 ent classes of Echinoderms between direct development 01 

 continuous growth, and a complete metamorphosis like that 

 of the star-fish or sea-urchin, in which the pluteus or larva 

 is but a temporary scaffolding, as it were, for the building 

 up of the body of the adult. 



Turning now to the classification of the Holothurians, 

 and beginning with the lowest, simplest, most generalized 

 forms (which are also remarkably worm-like), and ascend- 

 ing to higher or more complicated forms, we find that there 

 are two orders, those without feet (Apotla) and those with 

 ambulacral feet (Pedota).* 



* It is possible that the Holothurians should he divided into two sub- 

 classes, one Diplostomidea Semper, in which the body is spherical and 

 the mouth and anus are close together, with ten ambulacral rows, etc., 

 and the normal, cylindrical, bipolar Holothurians. Semper's Diplostomi- 

 dea is based on RJiopalodina lageniformis Gray, from the Congo Coast, 

 and regarded by Semper as the type? of a fifth class of EcliMiodcnns. 



