PART IV. 



THE MOLPADIID^]. 

 Order ACTINOPODA Ludwig. 



External appendages of the water-vascular system arise only from the well- 

 devoloped radial canals. 



Family MOLPADIID^E, J. Miiller. 



More or less elongated, rather stout holothurians, with an anterior, flat, 

 circular oral disc and generally tapering posteriorly into a more or less evi- 

 dent caudal portion ; with well developed respiratory trees, but with water-vas- 

 cular system greatly reduced; 15 (in only one species, 10) circumoral tentacles, 

 simple or digitate, usually with conspicuous ampullae, are present ; radial water 

 vessels are present, but, except for a few very rudimentary papillae at their 

 posterior termination, they are rarely associated with any outgrowths of the 

 body-surface; circular muscles are interrupted at each radius, and each radial 

 longitudinal muscle usually consists of two parallel bands; no special sense- 

 organs are present, nor are there ciliated funnels on the mesenteries or wall of 

 the body cavity ; calcareous deposits, commonly in the form of tables, fusiform 

 rods, or perforated plates, usually present; anchors sometimes occur, but wheels 

 and sigmoid bodies do not; phosphatic deposits (see page 143) often present. 



MORPHOLOGY. 1 



FORM AND SIZE. The body is generally of considerable diameter in propor- 

 tion to its length, the ratio ranging from nearly equal in some young speci- 

 mens to about 1 -.10 in some old and fully extended individuals ; excluding the 

 caudal appendage, the length is not often more than three or four times the 

 diameter, although of course much depends on the amount of contraction. The 

 caudal appendage is commonly quite evident (see Plates IX, X, XI, XIII), but 

 is entirely wanting in Acaudina and is more or less indistinct in several species 

 of other genera. It is commonly from one-twelfth to one-third of the total 

 length, but may be more than one-half in Caudina and rarely so in Molpadia. 

 The largest member of the family is apparently Aphelodactyla molpadioides 

 (Plate III), which is sometimes 210 mm. long and correspondingly stout; Cau- 

 dina arenata undoubtedly reaches a greater length, perhaps 250 mm. when fully 



'In the preparation of this section I am particularly indebted to Gerould's admirable report on 

 Caudina arenata (1896). which has never been surpassed as a study of holothurian morphology. 



135 



