456 REVISIOX OF THK AXIXELLIIVE, i. 



or they are closely arranged in subcircular, sieve-like groups, as 

 in T. reteporosns and T. pustidosus (PI. xxvi., figs. 4, 5, 7, <S; PI. 

 xxvii., figs. 5, 6). In the latter species, the surface presents a 

 minutely reticulate appearance. 



Dried specimens are whitish on the surface, owing to the 

 presence of a thin dermal crust of spinispiral microsL-leres; in 

 alcohol, the colour varies in the different species, from whitish- 

 grey to pale orange-yellow. The colour of li\ing specimens — 

 known so far only in the case of 7'. rrff-jtorosxt^, in which it is 

 brilliant orange, red or scarlet — is probably always to some 

 extent determined by, or dependent upon, that of a symbiotic 

 Myxoph3'cean alga, which appears to be invariably present in all 

 the species, often in enormous numbers. 



The main skeleton, which is composed of non-pluinosu spiculo- 

 spongin fibres, is almost exactly similar in its conformation (except, 

 presumably, in 7'. bacterium) to that described by Vosmaer* as 

 typical of the genus Axinella (s.str.). In the central region of 

 each branch, it forms an abruptly delimited dense core, or axial 

 fune, composed of ramifying and interuniting l<jngitudinal main 

 fibres additionally connected (more or less obliquely) by a greater 

 or lesser number of transverse fibres, and presenting (in longi- 

 tudinal section) a s(jmewhat lattice-like arrangement (PI. xxvi., 

 fig. 1); and extra-axially it consists mainly or almost solely of 

 very sparsely ramifyijig, radial fibres, which arising as branches 

 from the longitudinal fibres (usually at some distance within the 

 axial fune), run outwards to the surface at approximately equal 

 distances apart, and are connected, onlv at irregular and usually 

 distant intervals, by spongin-ensheathed single spicules and l)y 

 paucispicular fibres of a single spicule's length (PI. xx\-., fig. 1). 

 The fibres are composed chiefly or almost entirely of spicules, 

 which are arranged for the most part parallelly or nearly so 

 (though not, as a rule, very compactly nor in a very orderly 

 fashion); and this arrangement is maintained to the very ex- 



■■* Vosniaer, (4. C. J., "On the distinction between tlie genera A.vine.Ua, 

 Phaktilia, Acanfhel/a, etc. ' Zool. Jahrh. Suppl. xv., 1912, p.31U, PI. xvi., 

 ti>'.s.5, (5. 



