380 TROF. E. A, MINCHIN ON THE BRITISH [DeC. 13, 



length, reaching 120 yn or more, and tapering gradually to a sharp 

 point (text-fig. 94, fig. 10 j, p. 377 ; text-fig. 95, fig. 16 fZ, p. 379). 



(b) Monaxon Spicules. — Three kinds occur, two invariably, the 

 third only to be found in certain specimens. (1) The ordinary 

 monaxons (text-fig. 94, figg. 12e-12_/, &c., p. 377) are curved and 

 liave distinct lance-heads, and vary greatly in size, in the same or 

 in diflferent specimens, ranging from 80 to 320 /u in length and 

 2 to 9 /u in thickness ; they cannot, however, be divided into two 

 distinct classes as has been done by Haeckel, since every possible 

 gi-adation is found from the smallest to the largest. The shaft is 

 generally more or less straight for its proximal half or two-thirds 

 and then has a slight bend, the distal fourth or sixth, however, 

 with the lance-head, being nearly straight. The shaft is thickest 

 in the region between a third and a half of its length from the 

 proximal end. The proximal half or third is cylindrical and 

 tapers abruptly to a point ; the distal half or two-thirds tapers 

 gradually and almost imperceptibly, dwindling to about half the 

 greatest thickness when it reaches the small lance-head. (2) Very 

 slender monaxons occvir (text-fig. 94, figg. 10 k, 10 Z, 12 c, 12 d, 

 p. 377), easily found on account of their very refringent appearance 

 and sharp outline, though they are much less abundant than (1). 

 They have sti-aight, veiy slender shafts and distinct lance-heads, 

 and vary from 70 to 110 yu in length, never reaching a large size. 

 (3) In some specimens fairly large bayonet-like spicules occur 

 (text-fig. 95, figg. IQg, 17 a, 176, p. 379) with a sharp bend in the 

 middle, sometimes represented only by a swelling. They reach 

 about 200 ju in length, varying in thickness from about 7-10 fj.. 



Variations of the Spiculation. (1) As regards the relative numbers 

 of the Spicides. — In some specimens the triradiates and quadri- 

 radiates are all of the ordinary slender type [a, 1 & 4) and none 

 of the thickened T-shaped forms (as 2 & 5) are to be found, or 

 only after much searching. In others they are more easily found. 

 The brackets, with or without gastral rays {a, 3 & 6), are never 

 very abundant and often scarce. Of monaxon spicules, the barbed 

 forms (&, 1 & 2) are always present more or less abundantly, 

 but in some specimens I have not succeeded in finding the large, 

 bayonet-like forms {b, 3), in spite of much searching ; in others 

 they are easily found, and I have also come across one in situ in 

 the body- wall of the sponge. 



Explanation of Text-fig. 96 (opposite). 

 Spicules of Si/oon sp. and Leucosolenia variahilis, 



Figg. 20rt-20M. Spicules of a Sycon found commonly occurring in company with 

 L. variabilis, specimen from RoscofF. a-e, dermal triradiates ; f-J, tubar tri- 

 radiates ; k, I, quadriradiates from oscular rim ; m-p, gastral triradiates and 

 quadriradiates ; q-u, monaxons. — Figg. 21 a-p. Spicules of a specimen of 

 L. variahilis from Bergen, Norway, identified by Haeckel as Ascandra varia- 

 hilis (Berlin Mus. no. 417), showing admixture of Sycon spicules similar to those 

 in the preceding figure, a-c, triradiates of L. variahilis; (i-Zi, quadriradiates 

 of L. variahilis; i,j, monaxons of L. variahilis ; Tc, I, tubar triradiates of the 

 Sycon ; m-p, monaxons of the Sycon. 



