62 STARFISH GALLERY. 



are always comparatively short and delicate, as compared with tne 

 spines found at the sides of the groove in Starfishes. 



The principal types of this Order are exhibited in Case 4 ; the 

 most exquisite of them are the forms whose arms are divided and 

 subdivided till they end at last in the finest threads, as in Astro- 

 phyton, the so-called Basket-fish or Gorgon's heads. 



Echixoidea, or "Sea-Urchins," are Echinoderms in which the 

 rays are not free, as in the Starfishes or Brittle-stars, but unite to 

 form a compact, spherical, heart- or disk-shaped test ; this test is 

 covered with spines, which may attain to a great length, as is 

 shown in the fine example of Diadema setosum from the Andaman 

 Islands ; some of the tests are flexible and very fragile j the form 

 shown by a drawing in Case 5 c will be seen preserved in spirit in 

 \Yall-case IV. Owing to the quantity of specimens that are some- 

 times dredged at one spot, the naturalist has been able to gain a 

 better idea of the range of variation in the species of Echinoderms 

 than in some other divisions of the Animal Kingdom ; an instruc- 

 tive series, showing the variations of Echinometra lucunter, is shown 

 in Case 6 a. 



The genus Hemiaster offers an example of an Echinoderm in 

 which the eggs are laid in special pouches ; the hinder ambulacra 

 are deepened to form pits, which are guarded by specially elongated 

 spines (see Case 6f) ; in these pits the young pass through all the 

 stages of their development. 



The HoLOTHURioiDEA, or Sea-Cucumbers, form the last order 

 of Echinoderms. Their body, as indicated by their English name, 

 is elongate, subcylindrical, with a more or less flexible integument, 

 according to the extent of the reduction of the calcareous skeleton ; 

 the mouth is at one end of the body and surrounded by tentacles, 

 the vent at the opposite end. 



As these animals cannot be shown in a dried state, some of them, 

 preserved in spirit, are placed in Wall-case IV. According as they 

 have or have not the sucking-feet of the Echinodermata, they are 

 ordinarily divided into the Pedata and the Apoda -, the latter are 

 represented by Synapta, which may attain to a great length, and 

 by Cltirodota ; the Pedata are illustrated by the genera Cucumaria, 

 Psoitis, and Holuthuria. The deep-sea investigations of the last 



