120 SPONGES 



an irregular manner at a late stage in the life-history, setting a 

 limit to further growth. 



General Remarks on the Skeleton. Beautiful instances of adaptation to 

 the conditions of life in abyssal depths are seen in the arrangement of 

 the skeleton in sponges of this group. Thus in Euplectella the spicules 

 are arranged in fibres which run either longitudinally, or in transverse 

 circles, or diagonally, to form spirals running in two directions. The 

 longitudinal and transverse fibres strengthen the sponge to support the 

 weight imposed upon it by the continual shower of particles, skeletons of 

 Radiolaria, etc., raining down upon it from the surface. The spiral 

 fibres correspond to the lines of stress and strain produced in a cylinder 

 fixed at one end and free at the other, which is acted upon by a force at 

 right angles to its axis, and strengthens the sponge against the action of 

 currents. Some species of Euplectella are cornucopia-shaped and further 

 strengthened by lateral ridges (Fig. 15) ; such a form is adapted to 

 constant currents in one direction. Other species, adapted to currents in 

 any direction, are cylindrical and upright, and strengthened equally on 

 all sides (Keller, 1891). 



In a brief but suggestive memoir Schulze [22] has drawn attention 

 to the remarkable fact that although the spicules of Hexactinellids are 

 composed, apparently, of non-crystalline material (colloid silica), yet their 

 axes possess the same symmetry as the crystals of the cubic system. Not 

 only is this true of the ordinary hexactine, but it is also seen in many of 

 the less common forms of spicule. Thus the discoctasters are spicules with 

 eight rays terminating in discs, each disc corresponding in position to one 

 of the eight corners of a cube ; again, in the nodes of the dictyonal frame- 

 work of many forms (e.g. Aulocystis), the twelve edges of the regular 

 octahedron are marked out by girder-like trabeculae ; and the six 

 secondary planes of symmetry of the cubic system are often indicated by 

 branching of the hexactines, or by their hook -like curvature. These 

 facts invite a renewed investigation of the physical nature of the spicule 

 material ; should it prove beyond all doubt to be non-crystalline, then 

 these striking imitations of crystalline axes must be regarded as mechanical 

 adaptations in a supporting framework the culmination, rather than the 

 starting-point, of the evolution. 



3. Histology. The finer structure of the body wall is of very 

 uniform, and at the same time of very simple, composition. The 

 dermal membrane is covered by a flat epithelium, and the under- 

 lying parenchyma is composed as in other sponges of a matrix 

 containing collencytes, amoebocytes, and, doubtless, scleroblasts, 

 besides sperm masses and ova. A remarkable feature of the dermal 

 layer is its trabecular structure. Fine strands of tissue stretch 

 in every direction over a continuous lacunar space, furnishing a 

 very complete filtering apparatus for the ingoing water current. As 

 a consequence of this peculiar structure, the connective tissue system 

 is very greatly reduced in quantity, and in the trabeculae there 

 seems to be no sharp distinction between the epithelial and 



