REPORT ON THE HOLOTHURIOIDEA. 143 



membranous layer, which, is strengthened by some muscular elements and seems to 

 include some small cavities. 



As in all previously known Holothurids, the efferent duct is generally simple, but in 

 some cases it may be observed that its end, when piercing the perisoma, gives off some 

 very fine branches, each communicating with the surrounding medium by a pore. In 

 Elpidla purpurea and Peniagone vitrea the efferent duct divides into two narrow 

 divergent branches, which pass in opposite directions through the perisoma, and open 

 externally, one on each side of the madreporic pore. In Peniagone ivyvittii the efferent 

 duct (PI. XXXVII. fig. 6, e) is surrounded by the same thick and dense sheath of con- 

 nective tissue which envelops the madreporic canal, and divides into two short, wide, and 

 di vergent canals, each of which, when the inside of the perisoma is reached, terminates 

 in about eight long slender canals, which run within the perisoma, and communicate 

 with the exterior by pores. These pores are scattered, not only over the anterior portion 

 of the odd interambulacrum, but sparsely on the lateral interambulacra also. 



The walls of the reproductive organs are often strengthened by calcareous deposits, 

 which in some forms, as, for instance, Deima fastosum, &c, are very closely crowded, and 

 covering one another, the walls thus becoming very hard and brittle. Having nothing of 

 importance to add to the facts already known regarding the histological structure, I only 

 refer to Plate XXXVII. , which shows some sections of the reproductive organs. 



General Remarks. 



The most remarkable and distinguishing characteristic of the Elasipoda is their 

 agreement in several important points, in their inner as well as their outer organisation, 

 with the larval state, an agreement more close than occurs in any previously known 

 Holothurid. The following characteristics are especially worthy of note as reminiscences 

 of the development of the Holothurioidea : — 



1. The strongly marked bilateral symmetry of the body and the fact that the highly 

 convex dorsal surface is often extended further than the mouth, which thus becomes 

 fully ventral in position. 



2. The presence of pedicels on the ventral surface only, and their arrangement in pairs, 

 but, above all, in the Elpidiidaj, their small number and their occasional position on the 

 posterior part of the body only. 



3. The simple shape of the calcareous deposits of the body-wall. 



4. The simple conformation of the calcareous ring. 



5. The communication of the water-vascular system with the exterior. 



6. The absence of respiratory trees and ciliated cups. 



Our present knowledge of the development of the higher Holothurids is rather 

 unsatisfactory, and confined to that of a few forms. However, the development of 

 llolothuria tremula, Gunner, Holothuria tubulosa, Gmelin, and Cucumaria doliolum, 



