NO. 10 CRUSTACEAN METAMORPHOSES — SNODGRASS 4I 



the original cypris (B). Soon a new cuticle is secreted on the surface 

 of the sac (D, iCt) in continuity with the cuticle of the attached 

 antenna, and the larva becomes a compact oval body still within the 

 shell but now entirely free from it. Again, as if preparing for a moult, 

 a second cuticle (^Ct) is formed beneath the outer one, and a small 

 point (d) grows out from its anterior end into the hollow of the 

 antenna. The body of the larva then retracts within the outer cuticle 

 (E), and as it does so the cuticular point elongates into a long, hollow 

 dartlike tube (d) with the narrow end cut off obliquely like the point 

 of a hypodermic needle, and its widened base embedded in the body of 

 the retracted tissue of the larva. This newly formed organ Delage 

 called the dart, and the larva armed with the dart he termed a kentro- 

 gon (from Greek kentron, a dart, and gonos, a larva). The shell 

 together with its loose inclusions is now thrown off, leaving the kentro- 

 gon, still enclosed in the outer cuticle, attached to the erab by the 

 antenna (F). 



The body of the larva again expands and pushes the dart into the 

 antenna (fig. i6 F) until its tip comes into contact with the integument 

 of the crab. Since the parasite is held fast by the antenna, the dart 

 pierces the integument instead, pushing the larva away from it, and 

 finally (G) projects into the body of the crab. Now the soft tissues of 

 the larva contract away from the cuticle but remain still connected 

 with the base of the dart. The remains of the larva thus have a free 

 passageway into the body of the crab through the narrow channel of 

 the dart, the orifice of which is said by Delage to be 3 to 6 microns 

 in diameter. Though Delage says he did not observe the actual passage 

 of the larval substance through the dart, globules are seen inside the 

 dart and the parasite is next found inside the crab. By the method of 

 the Sacculina a mouse might get into the pantry through the keyhole 

 of the door, but once inside it would have to devise a new way of 

 eating. This problem the Sacculina solves very easily — it simply 

 adopts the feeding method of a plant by sending out absorbent roots 

 among the organs of the crab. 



Inside the crab the parasite becomes a small oval body consisting 

 of a mass of cells enclosed in an ectodermal epithelium. It finds its 

 way to the ventral side of the crab's intestine and here becomes at- 

 tached. Now the principal concern of the parasite is to obtain nourish- 

 ment from the host for maturing the germ cells which it has brought 

 with it from the cypris stage. Incidentally, this will be the first food 

 from an external source that the larva itself has had, since it was 

 hatched without an alimentary canal. The larval body expands 

 against the intestine of the crab (fig. 17 A) and sends out branching 



