REPORT ON THE HOLOTHURIOTDEA. 65 



The tentacles are generally equally large, though not rarely individuals occur in 

 which they are unequal, one or several being considerably smaller. It is necessary 

 to observe, that those smaller tentacles, which have often the same shape as the fully 

 developed ones, but which are sometimes much reduced and destitute of processes on 

 their usually somewhat swollen ends, frequently though not always belong to the 

 ventral surface ; the smaller individuals bear tentacles generally of unequal size. The 

 normal tentacle is dilated towards its end, which is discoidal or hand-like, and commonly 

 provided with six simple digitiform processes round its margin ; sometimes the processes 

 are seven or eight in number, as is the case with the individuals from Stations 157 

 and 299. The tentacle itself is not capable of being retracted, while its processes on 

 the contrary are retractile. 



As is above noted, the number of pedicels varies extremely, as does also their 

 position. In the large typical forms they are disposed in two distinct rows along each 

 side of the ventral surface, while this arrangement in double rows is scarcely dis- 

 cernible in other individuals, the pedicels being arranged in a single but irregular row 

 along each side. The pedicels of the inner row constantly alternate with those 

 of the outer, so that if we could imagine them united by a line, it would run in a 

 zigzag course. The inner pedicels, being always smaller than the outer ones, seem 

 to be developed later, which may be fully proved by the fact that they are generally 

 rudimentary in young animals, while those of the outer row are well developed. The 

 pedicels are supported by numerous calcareous deposits, are non-retractile, and reach 

 a considerable size, those of the largest individuals attaining a length of 16 mm. and 

 measuring in diameter at their base about 6 '5 mm. The odd ambulacrum bears some 

 pedicels, commonly three to four, but sometimes nine, which, in most cases are 

 rudimentary, and rarely acquire a more considerable size ; they always remain, however, 

 much smaller than the pedicels of the lateral ambulacra ; two of them are generally 

 situated side by side immediately in front of the anal aperture. As is shown in the 

 above table, the processes vary greatly in number and dimensions. They are disposed in 

 four single rows, two running along the dorsal ambulacra, and the others placed along 

 the sides of the body a little above the ventral lateral ambulacra with which they are in 

 connection. Thus the dorsal ambulacra as well as the lateral ambulacra of the ventral 

 surface are furnished with processes, the lateral ones proceeding from the lateral inter- 

 ambulacra. At the posterior and especially at the anterior extremity of the body, 

 where the ambulacra approach one another, it is difficult to decide from the position 

 of the processes which of the ambulacra they belong to, the processes being so closely 

 crowded at these places that it is only by means of injection or a close anatomical 

 examination, that certainty can be obtained. The size varies very greatly even in 

 the same animal, some of the processes being rudimentary while others attain a con- 

 siderable length almost equal to the breadth of the body. They are usually of a very 



(ZOOL. CHALL. EXP. — PART XIII. 1881.) N 9 



