The extrusion itself depends enFIG.
Proboscis is everted Partial extrusion lessening the resistor rhynchostome.
This was the result of several conditions, among them the recent development, through warping and faulting and volcanic extrusion, of high lands with more or less considerable slopes.
The basic lavas are usually darker and denser than lavas of acid type, and when fused they tend to flow to great distances, and may thus form far-spreading sheets, whilst the acid lavas, being more viscous, rapidly consolidate after extrusion.
Very few Crustacea are viviparous in the sense that the eggs are retained within the body until hatching takes place (some Phyllopoda), but, on the other hand, the great majority carry the eggs in some way or other after their extrusion.
In most Heteronemertines it is found to be an elongated slit with corrugated borders; in the Metanemertines it is smaller and rounded; in Malacobdella and Akrostomum it, moreover, serves for the extrusion of the proboscis, which emerges by a separate dorsal opening just inside the mouth.