160 HYALONEMA (HYALONEMA) OBTUSUM. 



longest is sometimes more than twice as long as the shortest. The greatest 

 difference in length between the lateral rays of the same hypodermal pentactine 

 observed was 310 m- The lateral rays are straight, conic, and blunt. They are, 

 in the large hypodermal pentactines of var. gracilis, 240-730 fj. long and 32-60 ^ 

 thick at the base. In the small ones they measure 135-440 ix in length. The 

 small hypodermal pentactines accordingly have, relative to the proximal ray, 

 considerably longer lateral rays than the large ones. At the end the lateral rays 

 are usually from one fifth to one third as thick as at the base, and here measure 

 5-22 IX in transverse diameter. The hypodermal pentactines of var. robusta 

 have a proximal ray 0.47-1.3 mm. long, and 40-80 yu thick at the base. The 

 lateral rays are on the whole attenuated towards the distal end less than in the 

 hypodermal pentactines of var. gracilis. They are, when not reduced, 250-750 n 

 long and, at the base, about as thick as the proximal ray. 



The end-parts of the lateral rays of these spicules exhibit remarkable irregu- 

 larities of external shape and internal structure. These irregularities are the 

 more conspicuous the thicker (the more blunt) the rays. Such an irregular 

 lateral ray-end of a hypodermal pentactine of var. gracilis is represented (Plate 

 33, fig. 17). A rudiment of a branch-ray, arising a short distance from the tip 

 of the main-ray, and a marked irregularity in the axial thread and the stratifi- 

 cation of the siliceous body of the latter are noticeable in this spicule. I am 

 inclined to ascribe these irregularities to the influence of the obstacles — other 

 spicules — met by these rays during their longitudinal growth. The cells 

 building the tips of these rays were forced to act in an abnormal manner; being 

 prevented by other spicules from adding to the length of the axial thread and from 

 depositing silica around it in a normal and regular manner, they produced the 

 irregular structures observed. The obstacles (other spicules) which thus cause 

 these irregularities are probably the stout proximal rays of adjacent hypodermal 

 pentactines. 



The hypogastral pentactines of the gastral cone of var. gracilis have straight 

 proximal apical rays, usually 240-400 n long, and about 12-20 n thick at the 

 base. The lateral rays are slightly inclined towards the proximal ray, straight, 

 and generally 180-250 n long. In the specimen of var. robusta the parts contain- 

 ing these spicules appear to have been lost. 



The hexactine megascleres of the distal part of the choanosome (Plate 34, 

 figs. 5-18, 19d) are in both varieties very variable in size and have a maximum 

 diameter of 350 /i - 2 mm. The rays of the same spicule are often unequal. The 

 greatest difference of length observed in them was 400 /*. The rays are 130-950 ix 



