193 ANATOMY AND PHYSIOLOGY 



fibre ; but the disposition of the network is not entirely 

 irregular, but like that of the latter genus, more or less 

 composed of a primary series of lines radiating from the 

 axis or base of the sponge, and of secondary series connect- 

 ing the primary ones at about right angles to them ; in fact 

 simulating very closely the arrangement of the skeleton of 

 Chalina oculata, 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 characters of a Halichondria is simu- 

 lated. 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 Isodidya. In a hasty examina- 

 tion 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 generally prevaihng character of fragihty ; the primary 

 lines being composed of very few spicula, while the secon- 

 dary 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. 



Isodidya infundihuliformis 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 difference 

 in the form of their spicula. In some species of the genus, 

 as in /. Simula, the cementing keratode of the skeleton is 

 so abundant in some parts as to cause it to simulate very 

 closely the structure of a Chalina, but the irregidarity and 

 compressed form of this pseudo-fibre is readily to be dis- 

 tinguished from true keratose fibre by a careful observer. 

 In other species, as in I. mammeata, the sarcode surrounding 



