SKELETAL STRUCTURES. 73 
and these again subdivide into minute twig-like extensions, which are beset with 
tubercular nodes and swellings. The spicular rays are usually subequal in length ; 
in each there is an axial canal, which in some cases subdivides, and extends into 
the main divisions of the ray, but cannot be traced into the minute extensions. 
Typical spicules of this family are present in the genera Siphonia, Park., Phyma- 
tella, Zitt., and the existing Discodermia, Bocage. In Plinthosella, Zitt., Spongo- 
discus, Zitt., and Phymaplectia, Hinde, the skeletal-spicules are less regularly 
developed, the rays are often very unequal in length, and diverge from each other 
at unequal angles, the branches terminate obtusely, and the entire spicule is thickly 
covered with rounded tubercles (Fig. 5, ¢). Some of the skeletal-spicules of 
Rhagadinia have one of the four rays reduced to a rounded knob, and the other 
rays are partially annulated. 
In many lithistid Sponges the spicules forming the stem and _ root-like 
appendages of the Sponge differ from those of the body-portion, the modification 
generally resulting in an elongation of the rays. 
Dermal Spicules of Lithistids—In addition to the skeletal-spicules, on the 
characters of which the four families above mentioned are based, there are also in 
most, if not all, lithistid Sponges, modified spicules forming the outer surface of the 
Sponge, which in many instances depart widely in form from the skeletal-spicules. 
Some of these are distinctly tetractinellid, and are precisely similar in form to the 
trifid spicules with horizontally-expanded head-rays, which have already been 
referred to (Fig. 3,d). Thus in the genus Doryderma, with its large, irregular 
skeletal-spicules, the dermal spicules are minute trifid spicules with short, simple, 
or furcate head-rays. The dermal spicules in some Rhizomorina Sponges are of the 
same general character as those of the skeletal forms, but of much smaller pro- 
portions, and with more closely arranged branches. In others, as in Pachinion, for 
example, these are mingled with trifid spicules with horizontally-expanded head- 
rays. ‘These latter are also present in the tetractinellid genus Callopegma, as well 
as in other Sponges of the same family. 
In other more specially modified dermal spicules, the primary head-rays of the 
trifid spicules become flattened out horizontally, and either divide and subdivide 
so as to form a filigreed expansion (Fig. 5, f), or they are united into a circular or 
lobate disc, in which the individual rays altogether disappear, though the relations 
of the spicule are still shown by the presence of three minute canals in its centre, 
indicating its origin from a simple trifid spicule (Fig. 5, h). In the filigreed forms 
the canals only extend to the first or second subdivision of the rays (Fig. 5, f). 
In these dermal spicules there is a minute central ray or shaft, which extends at 
right angles to the expanded head. The dermal spicules in the genus Plinthosella 
are minute, delicate, oval, lath-shaped or irregular lamine, in which neither canals 
nor vertical shafts are developed. 
K 
