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



3. Trichodragmata (woodcut, Fig. IV., 2) ; hair -like spicules arranged in more or 

 less comjjact bundles. Such forms are very common in the genus Es2)erella. 



4. Toxa (woodcut, Fig. IV., 4) ; bow-shaped spicules, tapering towards either 

 extremity and with the ends often spinose. These occur in many sponges. A slight 

 modification of the tyj^e is found in the " forcipiform " spicule of Halichondria 

 forcipis} In one sponge, Ampliilectus pilosus, nobis, we have found the toxa, which 

 in the young condition are very distinct and strongly curved, passing by gradual 

 transitional stages into simple, much elongated, slender oxea of such a size, that, did 

 they occur apart from the small toxa, they would certainly be classed amongst the 

 megasclera {vide PI. XIX. figs, ba-ba'""). This fact well illustrates the difficulty in 

 classifying spicules according to size and form. 



5. Toxodragmata (woodcut, Fig. IV., 5) ; more or less compact bundles of toxa, 

 which have aU developed in one and the same cell. 



B. Hooked Forms. 



1. Sigmata (woodcut. Fig. V., 1, 2) ; each consisting of a slender, cylindrical shaft, 

 which is curved over so as to form a more or less sharp hook at each end. The two 

 terminal hooks may curve both in the same direction, when the spicule is said to be 

 simple (woodcut, Fig. V., 2), or they may curve in different dii-ections, when it is said 

 to be contort (woodcut. Fig. V., 1). There is, however, no real distinction between the 

 two, and, as a matter of fact, the spicules are nearly always contort to some extent. 



Fig. v.— Hooked forms of microsclera. 



Sigmata are perhaps the commonest of Monaxonid microsclera, being very characteristic 

 of the Desmacidonidse. 



2. Sigmadragmata (woodcut, Fig. V., 3); more or less compact bundles of sigmata, 

 which have all developed in one and the same cell. 



3. Diancistra (woodcut. Fig. V., 4); hooked forms of very peculiar shape ; character- 

 istic of the subfamily Hamacanthiuse. The spicule resembles a large stout sigma, but 

 the inner margin of both shaft and hook thins out into a fine knife-edge, notched as 



^ Vide H. J. Carter, Ann. and Mag. Nat. Hist., ser. 4, vol. xiv. p. 247, pi. xiv. fig. 32 ; cf. also tlie same spicule in 

 Forcepia colonensis, torn, cit, p. 248, pi. xv. fig. 47. 



