ACTINOPODOUS HOLOTHUIUOIDEA. 91 



ambulacrum. A bundle of several (up to 13 or more) stone - 

 canals on each side of the dorsal mesentery. A bundle of very 

 numerous fine Cuvierian tubes about 3 mm. long, on the basal 

 common part of the respiratory tree. 



One specimen measured in life 21.5 cm. When contracted, 

 naturally much smaller, ± 8 cm. long. Skin flaccid and soft to 

 the touch. 



I have been able to make out very interesting changes in 

 the calcareous deposits of this variety according to age. I shall 

 begin with a description of the deposits in fully grown in- 

 dividuals. These have been already characterized by Ludwig, 

 but I find some differences in details. The most characteristic 

 and most numerous elements among them are the flat, some- 

 times more or less bent, granulated rods (textfig. 19 e — /). 

 Their sizes are 0.100x0.024— 0.120x0.024 mm. Most frequent- 

 ly their entire surface is finely granulated ; but often the granu- 

 lation may disappear, and then the rods appear smooth. Along 

 their edge on both sides, they send out short simple processes. 

 Very often these processes bifurcate and may unite with 

 similar neighboring processes, producing a series of small holes 

 near the edge. All these characters appear in various com- 

 binations and permutations. Among these smooth rods ara 

 occasionally found. Ludwig states that smooth rods are found 

 only on the ventral perisome, but if what I call smooth rods 

 are the same as what Ludwig refers to, they are found also in 

 the dorsal. They are entirely smooth, and much smaller than 

 granulated rods. Their size is at the most ± 0.10 mm., and may 

 be 0.07 — 0.06 mm. Typically spindle-shaped, they may some- 

 times be simple rods, or such with the two ends slightly 

 bifurcated. They are never numerous ; on the contrary, they 



