xvi THE VOYAGE OF H.M.S. CHALLENGER. 



2. Tornota (woodcut, Fig. I., 2) ; the tornote form differs from the oxeote in having 

 the ends abruptly and suddenly pointed. 



3 



c 



<- > C 



Fig. I. — Diactinal megasclera. 



3. Strongyla (woodcut, Fig. I., 3) ; here the spicule is not pointed at all, but the 

 ends are evenly rounded off. 



4. Tylota (woodcut, Fig. I., 4); in these forms the spicule has a swelling at each 

 end, so that it is divisible into a central, elongated, cylindrical shaft and two terminal 



heads. 



B. Monactinal megasde7-a. 



Of these we need only distinguish two fundamental t}^3es : — 



1. Styli (woodcut, Fig. 11. , 1); pointed at one end and evenly rounded off at the 

 other without any swelling. 



Fig. II. — Monactinal megasclera. 



2. Tylostyli (woodcut, Fig. II., 2) ; in which a head is developed at one end of the 



spicule. 



In the case of monactinal spicules it is convenient to distinguish between a " base," 

 which is the blunt end of the spicule, and an " apex," which is the pointed end. 



C. Branched megasclera. 



We have now to speak of certain rarely occurring branched forms, which constitute 

 an exception to the general rule that the spicules of the Monaxonida are uniaxial. The 



Fig. ni.— Branched forms of megasclera. 



angle which the branches make with the shaft may be either greater than 90°, as in 

 woodcut. Fig. III., 1, or less than 90°, as in woodcut. Fig. III., 2. 



1. Cladostrongyla (woodcut. Fig. III., 1); when the spicule is branched at one end and 



