SKELETAL STRUCTURES. 81 



In Plinthosella, Zitt., and Phymaplectia , Hinde, the spicules unite together by the 

 interlocking of the tubercles on their lateral surfaces (Fig. 5, e) as well as 

 terminally, without forming prominent nodes, thus showing an approximation to 

 the mode of union in the Rhizomorina family. 



In the Megamorina family there are two distinct modes in which the skeletal- 

 spicules unite together, and both may take place in the same Sponge. In one, the 

 spicules and their branches are twisted round each other almost in the same manner 

 as the strands of whipcord. The spicules may be merely twisted at their ends, or 

 throughout their length ; and owing to the lateral and terminal notches (Fig. 4, g) 

 they are very closely fitted together, so as to produce a fibrous meshwork. This 

 mode of union is typically shown in Garterella, Zitt., and Isorhaphinia, Zitt. The 

 second mode is exemplified in spicules, in which the branches terminate in flattened, 

 concave, spoon-shaped expansions (Fig. 4, e), which closely and evenly fit, and 

 clasp the surfaces of adjoining spicules, and thus form a meshwork of open irre- 

 gular interspaces. This kind of union is well shown in Dory derma, Zitt. (Fig. 

 4,/), Heterostinia, Zitt., and in the recent Lyidium, Os. Schmidt. 



In the Anomocladina family the union of the skeletal spicules takes place, in 

 some respects, in the same manner as in the Megamorina family described abovej 

 that is to say, the terminal ends of the spicular rays are similarly furnished with 

 expanded surfaces (Fig. 5, a, b), which are firmly attached to the central nodes, 

 and occasionally to the rays of adjoining spicules, thus forming a mesh apparently 

 composed of star-shaped bodies, whose rays are all united together (Fig. 4, h). 

 These stellate bodies of the connected skeleton are thus of a compound character, 

 for in each there is the node and the rays proper to it, forming the elementary 

 spicule, and also other rays belonging to adjoining spicules whose terminal expan- 

 sions are firmly attached to the node. This union is usually so close that it is not 

 practicable to determine in the connected skeleton the rays proper to the node 

 from those which are merely adpressed to it (Fig. 4, h). In some instances, in the 

 skeleton of Astylospongia, the spicular rays converge to a point in which no central 

 node is present for them to clasp, but they terminate in precisely the same way as 

 if the node were in the proper position, and are thus grouped round a sub-spherical 

 cavity. In some examples of Cylindrophyma, the rays project from the node at 

 right angles to each other, and the connected skeleton has quadrate or subquadrate 

 interspaces, singularly resembling the mesh of hexactinellid Sponges, for which 

 it has sometimes been mistaken. Typical examples of the skeleton of Anomo- 

 cladina Sponges are shown in Cylindrophyma, Zitt., Astylospongia, Ferd. Roem. 

 (Fig. 4, h), and the recent Vetulina, Os. Schmidt. 



The skeletal-spicules of the Rhizomorina family are united together by the 

 close adpression of the minute facets terminating the numerous branches and 

 spinous processes of the spicules, to the main axis and branches of adjoining 



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