68 SPOLIA ZEYLANICA. 



There are small scattered papillae on the bivium and more 

 numerous and larger pedicels on 'the trivium, so that the two 

 surfaces are clearly distinguished. In the living condition 

 the back often appears almost smooth owing to the insigni- 

 ficant size of the papillae. This species may attain a length of 

 350 mm., but it generally contracts to half its length when 

 preserved. 



Internal Structure. — The calcareous ring is of the usual 

 aspidochirote type and is not very large. This form is 

 interesting, because the number of Polian vesicles and stone 

 canals varies in different individuals, the number generally 

 being much higher than in most Holothurians. I have 

 counted from one to five Polian vesicles and up to twenty-seven 

 stone canals. The circum-oesophageal ring is some distance 

 behind the calcareous ring. The right respiratory tree 

 extends up to the calcareous ring, but is very deUcate ; the 

 left respiratory tree is shorter, but much more bulky. I have 

 examined a great number of hving specimens but have never 

 seen any Cuvierian organs. The collected evidence goes to 

 show that these organs are absent in the above species. In 

 one specimen a small crab was found in the oesophagus at the 

 level of the calcareous ring. Whether it had been taken in 

 with the sand which passes through the alimentary canal, or 

 whether it was living commensally, I cannot say. 



Spicules. — The deposits of this well-known form consist of 

 tables and somewhat irregular perforated plates. The tables 

 are almost square in plan view and are 50 [jl in diameter and 

 60 t^ high, and generally consist of four large central holes 

 with a small hole at each of the four corners. Occasionally, 

 however, there may be an almost complete ring of small holes, 

 and the edge of the disc may be spinous. The tower consists 

 of four uprights joined by a transverse beam, and having four 

 sets of well-developed spines on the summit. The perforated 

 plates vary in shape and are not more than 25 [x in diameter. 



Remarks. — The two forms of H. atra which Theel and 

 Bedford regarded as distinct differ in respect of the spicules 

 and colour. The one form, H. atra var. amboinensis, is black 

 and has spines on the edges of the tables. The other, H. atra. 

 is black but may have the papillae and tube feet with whitish 



