120 



Prof. S. J. Hickson. 



bilateral symmetry, spicules are entirely absent in some species, but present 

 and very variable in others. 



In the radially symmetrical Veretillidse the spicules are so variable that 

 they afford very unreliable characters either for generic or specific diagnosis. 

 In Veretillum itself the spicules of the rachis are nearly all thin, flat plates 

 or rods, but their size and outUne shape are so variable, even in a single 

 preparation, that it is difficult to find any common characters. They may be 

 of almost any size up to a maximum of about 0'25 mm. in length. Some are 

 round or oval discs with serrate edges, some are oval or dumb-bell-shaped, 

 divided by a line into two parts, technically known as twins, others divided 

 into four parts by crossed line (the quadruplets), then there are straight rods, 

 spindles, and others of quite irregular outline. 



In a careful investigation of the Veretillum cynomormm of the Mediter- 

 ranean Sea, Medermeyer* finds that " Die aussere Form der Spicula ist 

 ausserordentlich variabel, und man kann wohl sagen dass sich kaum zwei 

 gleiche vorfinden." 



The variability of these spicules in form and size is paralleled by their 

 variability in number. In some specimens of a species the spicules are 

 so scarce that they may be overlooked, in others they may be very numerous. 

 Moreover, in some parts of the rachis of a single colony they may be crowded 

 together, and in other parts very scarce. 



In Lituaria we also find a great variety in the distribution and shape of 

 the spicules. Many varieties of flat plates and rods are found, similar to 

 those of Veretillum, but, in addition, we usually find a number of thick 

 double-star or " capstan " spicules, which occur in no other genus of the 

 order. 



In the wide-spread genus Cavernularia most of the spicules in some species 

 are straight, flat rods, but forked or branched forms and spindles are some- 

 times abundant. Ktikenthal and Brochf have placed a species into a 

 separate genus (Cavernulina) on the ground of the variability of its spicules, 

 and state, " mit voller Sieherheit," that such a variation does not occur in 

 Cavernularia. But this statement is not in accordance with the previous 

 investigations of KollikerJ nor of Balss,§ who states, in his description of 

 C. habereri, that the spicules are so extraordinarily variable that they are not 

 adapted for species diagnosis. 



* Niedermeyer, ' Zool. Anz.,' vol. 43, p. 263 (1913). 



t Kukenthal and Broch, ' Wiss. Ergeb. d. Deutschen Tiefsee-Expedition " Valdivia," ' 

 1911. 



X Kolliker, ' Abh. Senckenb. Ges.,' 1870. 



§ Balss.' Abh. K. Bayr. Akad. d. Wiss.,' Supplementband, 1910. 



