ON SOME PLYMOUTH HOLOTHURIANS. 221 



are more or less ellipsoidal in outline and larger than the former : in 

 other individuals, however, the relative sizes of these foramina are 

 reversed (see Brady and Robertson, -4, Plate LXXII, Fig. 3). 



The chief body spicule in C. normani is also plate-like but ellipsoidal 

 in outline ; it has rarely more than four foramina, and bears on each sur- 

 face usually 12 rounded bosses or nodules. The foramina are arranged 

 diamond- wise along the long axes of the spicule. There are a great many 

 more spicules in a given area of the body w'all in C. normani than in 

 C. saxicola, as may be easily seen in preparations of the skin : it is difficult 

 to obtain exactly comparable specimens, but at a rough computation one 

 would doubtless be well below the actual proportion in stating that they 

 are twenty times more numerous in C. normani than in C. saxicola. 



The surface body spicules in C. saxicola are well shown in Brady and 

 Robertson's figure (4, Plate LXXII, Fig. 2). Theyare microscopic, stellate, 

 of varying shape and size, but rarely more than 30 /x. wide, and scattered 



Fig. 6. — A single tube-foot of C. saxicola, showing the kind and number of the podial 

 spicules (x about 30). 



sparsely over the body. Usually they consist of a thin central plate 

 from which radiate tiny cylindrical rods about 12 yu. long. On the other 

 hand, the corresponding spicule in C. normani is bell-shaped, being 

 slightly rectangular across the mouth of the bell,- where on the average 

 they measure about 40 jj.. by 36 /x. These spicules are almost uniform 

 in size, forming a continuous covering over the whole of the body and 

 passing on to the bases of the tube-feet. Their compactness may be 

 gathered from Fig. 8, which is a view through a low power of a micro- 

 scope of a portion of the body wall of a small specimen in which, however, 

 only one of the bell-shaped spicules is fully developed. 



The podial spicules in C. saxicola are, as Pace has shown (loc. cit.), 

 usually perforated with a single series of holes. These are well shown 

 in Fig. 4, which is a drawing of a whole tube-foot well expanded (taken 

 from specimen 4-5, Table 1). One of the microscopic surface spicules 

 only is present. In C normani, on the other hand, these spicules are 

 mostly larger, and with two or more series of foramina (see Fig. 5, p. 219). 



