REPORT ON THE HOLOTHURIOIDEA. 127 



are of the highest interest. The composition of the calcareous ring in the Pedata and 

 Apoda being already wed known it is unnecessary to enter into particulars. The ring is 

 composed of ten or more usually solid pieces or ossicles of a net-shaped structure, the 

 five pieces of which, corresponding to the longitudinal muscles, are termed radial. In 

 very few cases as, for instance, in Embolus, Selenka, the calcareous ring is entirely 

 absent, and sometimes it is more or less rudimentary. 



In the Deimatidse the calcareous ring is made up of a fragile spongy network. 

 When trying to isolate the ring from the surrounding tissues it is very often 

 spoiled because of its fragility. In Lcetmogone the ring (PI. XXXVII. fig. 11) seems 

 to constitute a thin continuous network without any visibly separate ossicles and 

 with its exterior part presenting numerous larger or smaller wrinkles. By treating 

 it with a solution of potass, the ring is usually broken in pieces, but five large 

 irregular fragments are commonly left, which are the radial ossicles. To judge by 

 the insufficient material which has been at my disposal, the calcareous ring in Ilyodccmon 

 maculatus is constructed in the same manner as that of the former species, though it 

 differs by being perforated for the passages of the ambulacral nerves and vessels. In 

 Oneirophcmta mutabilis the ring is more plainly made up of radial and interradial 

 pieces, the radial ones being comparatively solid, of a more definite shape, and notched 

 instead of being perforated (PL XXXVII. fig. 4). The interradial pieces, fifteen (?) in 

 number, are more fragmental, cup-shaped, and extremely fragile. The calcareous ring 

 (PL XXXVII. fig. 3) seems to be of the same construction in Deima as in Oneiro- 

 phanta, though possibly more fragde. 



Amongst the three families which belong to the order Elasipoda, the Deimatidse 

 and particularly the genus Oneirophanta bear the strongest resemblance to the Apoda and 

 the Pedata with regard to the composition of the calcareous ring. The Psychropotidse 

 seem to form a transition between the Deimatidse and the Elpidiidse, but, unfortunately, 

 I have not had the opportunity to study the structure of their calcareous ring more in 

 detad. Most of the representatives of this family being in a highly macerated 

 condition, the calcareous matters being almost dissolved, and the individuals which 

 remained in an uninjured state being very few in number, any more detaded examina- 

 tion has been impossible. Though the imperfection of our knowledge at present 

 precludes positive assertion, there is every reason to believe that the calcareous ring in 

 the family in question must be highly undeveloped, and made up of an extremely thin 

 and fragile network which does not always form a continuous ring, but, as in 

 Euphronides depressa is composed of five small pieces separated from one another. 

 From what I have been able to observe in the Psychropotidse, each piece seems to bear 

 a certain resemblance to the spicules which compose the ring in some of the Elpidiidse, but 

 with the difference that, instead of a few, a great number of rods are present, which 

 anastomose with one another and form a network. 



