THE APODOUS HOLOTHURIANS 157 



a rosette from which the anchor rises. Phosphatic deposits of a yellow, brown, 

 or deep-red color are very commonly present. 



This large and widespread genus is most unsatisfactorily known and we 

 are only just beginning to realize the difficulty of distinguishing real specific 

 limits within it. Of the 27 species here listed only seven are known from any 

 considerable number of specimens, while of the remaining 20, six are known 

 from only a single specimen each, and eight others from fewer than five speci- 

 mens. Some will probably prove to be forms (perhaps different ages) of a 

 single species, while in other cases, such as musculus, there are probably several 

 distinct species included under one name. There is room for wide difference 

 of opinion as to the validity of many species here admitted, and of many of the 

 combinations of species, previously held to be distinct, here made. There can 

 be little question, however, that, in some of the species at least, the presence of 

 anchors is associated with immaturity and that with increasing age there is a 

 gradual transformation of the calcareous deposits, beginning with the anchors 

 and rosettes, into the peculiar, colored, phosphatic bodies. Of the significance 

 of this change we know nothing. (For a full discussion of the evidence, see 

 antea, p. 18). The species of Molpadia occur only in deep water and on muddy 

 or fine sand bottoms. The bathymetrical range is from 35 to 3,900 m., but 

 they rarely occur in less than 200 and not commonly in more than 2,000 m. Ap- 

 parently cold water and a soft bottom are essential to them. It would be well 

 if we could leave out of account in the following key all reference to the an- 

 chors and their plates, but in the present state of our knowledge that cannot be 

 done. They have, however, been ignored as far as possible. I have been un- 

 able to find any descriptions in any of VerrilFs publications of three species 

 attributed to him, namely, Trochostoma abyssicola, Trochostoma ayresii, and 

 Ankyroderma limicola. They seem to be "nomina nuda" and are consequently 

 ignored. I am unable to identify Cuvier's type species (holothurioides) with 

 any of the species known today, but I have very little doubt that the specimen 

 was an example of musculus. As there is room for doubt, however, it seems to 

 me unwise to attempt to replace the universally used name by the earlier one. 

 On the other hand, I cannot include Cuvier's species in my key or other- 

 wise, for from a modern point of view it has no distinctive characters. I have 

 therefore placed it doubtfully as a synonym of musculus. In using the follow- 

 ing key it should be borne in mind that young individuals of any species may lack 

 phosphatic deposits, while very old specimens may lack calcareous deposits. In 

 at least one species (musculus as here defined), and possibly in others, phosphatic 

 deposits appear to be absent at times in otherwise normal adults. More than 

 ordinary care should therefore be exercised. 



