46 C. F. JENKIN. 



not lie in one plane, foreshortening always produces an apparent distortion which 

 affects both the relative length of the rays and the angles between them. No attempt 

 has in general been made to correct the measurements for this ; the figures given are 

 the apparent lengths and angles as the spicules lie in various positions on the slide. 

 The distortion is large in such cases as Streptoconus australis, where the folding 

 angle is 120. 



The position of the spicules relatively to the other parts of the sponge body is 

 of some interest. It may be stated as a general rule, and probably as a universal rule, 

 that spicules do not pass through flagellated chambers. They usually lie entirely 

 within the mesoderm, except such parts as project beyond the dermal or gastral 

 layers. They occasionally project into, or cross, the incurrent and excurrent passages. 

 They appear in* these cases often to be surrounded by a considerable amount of body 

 substance ; if this is covered with an epithelial layer of cells the spicules, strictly 

 speaking, still remain within the mesoderm. The minute spicules (" Mortar spicules," 

 Haeckel), which in Leucandra often appear to be scattered irregularly through the 

 whole body-wall, all lie in the mesoderm surrounding the flagellated chambers, and 

 thus build up what may be described as a flagellated chamber skeleton (see Figs. 127 

 and 129). The very thin hair spicules, which extend in straight lines through the 

 body-wall of some species, all lie within the mesoderm. The enormous projecting oxea, 

 which pierce the body-wall at all angles in some species and appear to be quite 

 independent of the flagellated chamber structure, all lie in the mesoderm between 

 these chambers. Similarly the regularly arranged tubar or body skeleton lies in the 

 mesoderm. 



The complex adjustment between the different spicules and flagellated chambers 

 must be arrived at during the growth of the sponge. It therefore seems probable 

 that the longer spicules are formed before the flagellated chambers and are pushed 

 about by the latter as they grow. The mortar spicules, on the other hand, are 

 probably formed after or during the growth of the flagellated chambers. 



r spicules. The Greek letter T has been used to designate malformed triradiatc 

 systems in which the paired rays are in line. 



T spicules. The Greek letter T has been used to designate a type of oxeote 

 spicule in which the end is bent over sharply through about a right angle. 



The hastate ends of the oxea in Leucosolenia discoveryi* amongst which T 

 spicules are found are formed by two angular bends close together in the axis of the 

 spicule. The r spicules may arise owing to the absence of the second bend. 



Chiactines. -The facial rays of these spicules are very similar to those of 

 ordinary quadriradiates. The basal ray, which is the longest, is usually straight, but 

 sometimes bent slightly at a point not far from its junction with the paired rays (see 

 Fig. 76) and is round in section. The paired rays are usually slightly curved and are 

 often oval in section, being flattened in the facial plane. Viewed along the axis of the 



* r spicules also occur in Leucotolenia minchini and Leucosolenia lieberkuehni. 



