1920] Skeletal Element in Lithistid Sponges 55 



The clesma is a silicious body, in most species of a complexly 

 branched shape (Figs. 3, 4), formed by the continued deposition of 

 silicious material on a silicious spicule which we may call the basic 

 spicule, technically crepis (Fig. 1). The basic spicules, which are 

 especially present in growing parts of the sponge, are free bodies, 

 that is, they lie in the living tissues of the sponge unconnected with 

 one another or with other skeletal elements. The desmas on the con- 

 trary as they assume the final shape, become articulated with one an- 

 other, and in most species become firmly united to form a coherent 

 skeleton which presents the appearance of a network of silicious beams. 



Now the basic spicule of the desma is in some lithistida a four- 

 rayed spicule (tetraxial or tetractinellid spicule), in others a simple 

 rod-shaped spicule (monaxial). Different as are the basic spicules in 

 these two groups of species, the complete desmas are not always easy 

 to distinguish, for in both cases they may become complexly branched 

 bodies. This superficial similarity of the two kinds of desmas, those 

 built on tetraxial, and those built on monaxial spicules, is in itself 

 significant, but when we find, as we do in some species, desmas of the 

 latter class varying toward desmas of the former class, it becomes 

 clear that the desma built on a four-rayed spicule (tetracrepid 

 desma) is the original or ancestral type, while the desma built on a 

 monaxial spicule (monocrepid desma) is a derived type. These in- 

 teresting and important variations were first described by 0. Schmidt 

 in Discodermia clavatella (0. Schmidt, 1879, pp. 12, 24). Sollas 

 (1888, p. 341) confirmed the facts and convinced himself "that a com- 

 plete series of transitional forms connect the monocrepid and the tetra- 

 crepid desmas." Topsent (1904, p. 60) has discovered the same state 

 of affairs in another lithistid sponge, Macandrewia azorica. 



The four-rayed shape may thus be regarded as the original, actual 

 or ancestral, shape of the spicule which is transformed by the depo- 

 sition of silicious matter into the desma. This spicule in the case of 

 tetracrepid desmas in general is what is called a calthrops, viz., a 

 spicule in which the four rays are similar, making the same angle 

 with one another and having the same length. But in at least one 

 species, Desmanthus incrustans Topsent, an evolutionary change has 

 occurred, whereby one of the rays of the basic spicule has become 

 longer than the others, the spicule thus being converted from a 

 calthrops into a triaene. In the triaene, a very common form of spic- 

 ule in the non-lithistid tetraxial sponges, we distinguish, then, one 



