AND PHYSIOLOGY OP THE SPONGIAD^. 
1115 
Halichondria and CJialina, as defined in the present work. Like the former, the spicula 
of the network composing the skeleton are merely cemented together, not enclosed 
within a regular horny fibre ; but the disposition of the network is not entirely irregular, 
but like that of the latter genus, more or less composed of a primai’y series of lines 
radiating from the axis or base of the sponge, and of secondary series connecting the 
primary ones at about right angles to them — in fact simulating very closely the arrange- 
ment of the skeleton of Chalina ocidata, but without the keratose fibre surrounding the 
spicula of the skeleton in that sponge. 
In some of the species of this genus the symmetrical arrangement of the lines of the 
skeleton is distinct only near the surface of the sponge, while in the more deeply seated 
parts the irregular character of a Halichondria is simulated. In determining the 
species of this genus, the sponge requires to be carefully examined by sections at right 
angles to the surface, where the distinctive character rarely fails to be readily detected. 
On the contrary, in Halichondria panicea, the type of that genus, I have never succeeded 
in finding such a linear arrangement of the skeleton as marks that of Isodictya. In a 
hasty examination a single linear series of spicula will therefore often prove an excellent 
guide to the discrimination of this genus. 
In most of the species with which I am acquainted there is a genei’ally prevailing 
character of fragility — the primary lines being composed of very few spicula, while the 
secondary ones are most frequently unispicular. Most of the species are thin-coating or 
encrusting sponges, and rarely appear to rise in tuberous masses, as the numerous 
species of Halichondria are in the habit of doing. 
Isodictya infundibuliformis is perhaps the most perfect type of the genus, as in it we 
have the primary and secondary lines of the skeleton distinctly separated by the differ- 
ence in the form of their spicula. In some species of the genus, as in I. simulo, the 
cementing keratode of the skeleton is so abundant in some parts as to cause it to simu- 
late very closely the structure of a Chalina ; but the irregularity and compressed form of 
this pseudo-fibre is readily to be distinguished from true keratose fibre by a careful 
observer. In other species, as in I. mammeata, the sarcode surrounding the skeleton is 
so abundant as to cause it to simulate a delicate form of Chalina ; but on immersion in 
Canada balsam the fibre-like form disappears, the sarcode contracting into a mere granu- 
lated coating, and the skeleton assumes the normal appearance of Isodictya. Plate 
LXXIV. fig. 8. 
Spongilla, Linnseus, Lamarck, and Johnston. 
Halichondria, Fleming. 
The structural peculiarities of the skeleton of Spongilla are the same as those of 
Isodictya ; and if there had not existed a striking distinctive difference in their repro- 
ductive organs, the two genera must have been united. Under these circumstances I 
propose the folloiving as the characters of the genus Spongilla. 
Skeleton without fibre, composed of a symmetrical network of spicula; the primary 
