XXXIV 
THE VOYAGE OE H.M.S. CHALLENGER. 
Some clue to the nature of this disappearance is possibly furnished by the Stellettid, 
Myriastra clavosa. In this sponge an oscule constantly exists (PL XII. fig. 40) ; but in 
many cases its aperture is so much diminished by the growth of its margin towards the 
centre as to be invisible to the unaided eye, or even when examined under a simple lens ; 
indeed recourse must be had to serial sections for its discovery. The existence of a main 
excurrent canal can also only be ascertained by means of such sections. In Anthastra 
communis, a sponge very similar in general character to Myriastra clavosa, all traces 
of an oscule have disappeared, and the main excurrent canals cannot be traced to any 
common point of union beneath the cortex. 
A comparison of the two sponges leads to the suggestion, that in instances of 
Myriastra clavosa with a very small oscule, we have an indication of a passage towards 
the condition presented by Anthastra communis. The chief excurrent canals in the 
former extend at their centrifugal ends very near to the ectosome, a little more and they 
would penetrate it, and open to the exterior. If this should take place a second oscule 
would be produced, and if it were closed by a sieve-plate, as it probably would be, 
it would be indistinguishable from the pores ; the main branches of the excurrent canals 
might similarly penetrate to the exterior and several oscules result ; the condition of 
things presented by Anthastra communis would thus be brought about. 
Unless the original oscule of a young sponge continues to increase in size with the 
growth of the sponge, some such change in character as the foregoing might almost be 
predicted, and in Myriastra clavosa, so far from increasing it actually diminishes in many 
cases. The result of this is to lead to an increase in the water-pressure on the walls of 
the excurrent canals, and knowing how amenable tissues of all kinds are to the pressure, 
it will not surprise us to find an elongation of the centrifugal ends of the excurrent 
canals taking place in consequence. 
In many cases the oscule is not lost, so much as transformed, its originally simple 
opening being replaced by a pore-sieve (Cydonium) •, this should rather be termed 
cryptostomy than lipostomy. In those two remarkable sponges, Tribrachium schmidtii 
and Disyringa dissimilis, the cloaca is produced beyond the sponge-body into a long 
cloacal or excurrent tube, the walls of which are cribriporal, the pores replacing the 
oscular opening (PI. XVII. fig. 1 ; PL XLI. figs. 1, 5). 
Secondary Canals or Epochetose System.^ 
Folding is not confined to the choanosomal plate or spongophare, but frequently 
affects the entire sponge, producing secondary cavities or canals (epochets). The simplest 
case of an excurrent epochet — or exochet, — as we may term it for brevity, is produced by 
an incurving growth of a fan-shaped plate till the lateral edges meet and unite, a vasiform 
^ oyiiTo;, 0, a channel. 
