128 PARASITES. 



IV. — Parasites. 



A very common parasite in the Nemerteans is a Gregarina, which frequents the ali- 

 mentary chamber of Jmpkiporus lacti/Zoreus, according to Mr. Lankester, and is found abund- 

 antly in the same region of Lineus gesserensis and its allies. The presence of such animals in 

 the Nemerteans appears to have been first noticed by Dr. G. Johnston, who in 1837 described 

 them in L. gesserensis (with an accompanying figure) in the first volume of the ' Magazine of 

 Zoology and Botany/ thus :— cc When pressing a portion of the body between the plates of glass, 

 I have occasionally seen some bodies escape, of a curved fusiform shape, acute at both ends, and 

 marked with a pale circular spot. They have shown no signs of life, nor can I say what they 

 are, though it has occurred to me that they may be the embryo-young ; and that the worms may 

 in fact be ovo- viviparous." This excellent naturalist thus misinterpreted their true character. 

 Prof. Kolliker 1 in his contribution to the genus Gregarina, in 1848, more clearly defines their 

 nature, and describes them under the name Gregarina Nemertis, from the alimentary canal of 

 Nemertes delineatus {Folia delineata, D. Ch.). Prey and Leuckart, Max Schultze, Van Beneden, 

 and other authors have also noticed their presence. 



The Gregarince occur in swarms in many examples, and consist of elongated comma-shaped 

 bodies (Plate XIX, figs. 10 and 11), having a transparent investment filled with minutely 

 granular contents, and each has a large pale nucleus, measuring from rsW^h °f an i ncn upwards, 

 according to the size of the specimen. The nucleus shows faint markings when the parasite is 

 first extruded, but a distinct nucleolus is not very apparent, though from the recent excellent 

 observations of Ed. van Beneden, 2 it is probably present. In perfect specimens the snout is 

 pale, very faintly granular (and quite diaphanous), bluntly rounded, and marked by a slight 

 swelling of the body at its base, from which prominence the snout gently tapers. There is no 

 trace of rough points or other apparatus for attachment. Sometimes, as when the investment has 

 received injury, the surrounding water seems to pass inwards and separate at certain parts the 

 contained granules from the sheath, a fact which shows a certain degree of cohesion in the 

 contents in situ, or the presence of another layer. A favourable opportunity of examining the 

 parasites is occasionally afforded by the spontaneous rupture of some of the Nemerteans. The 

 Gregarince then project from the granular parenchyma throughout their entire length, with the 

 exception of the snout, by which they adhere. Indeed, this may often be seen in the perfect 

 worm, for the waves of fluid bend hither and thither the free bodies of the parasites. After 

 remaining for some time in the previously mentioned position (under pressure) a few separate 

 themselves, and move through the salt water with a slow gliding motion like that of a diatome. 

 On careful scrutiny the contour of the snout in a living specimen is observed now and then to 

 vary. The motion of the body is not due to currents between the glasses, as it passes through 

 mucus in the same manner. After remaining in salt water for eight or ten hours all movement 

 ceases, and in some the body becomes club-shaped (Plate XIX, fig. 11) ; at the same time the 

 clear portion at the snout is almost obliterated by encroachment of the granules. Occasionally 

 one of the Gregarina is observed in a degenerating condition, forming an ovoid body in which 

 the bent and atrophied parasite is scarcely distinguishable. 



1 c Zeitsch. fur wiss. Zool./ Bd. I, pp. 1 and 2, Taf. I, fig. 4 b. 



2 < Bullet, de TAcad. Roy. de Belg./ 2me ser., tome xxxi, No. 5, 1871. See also ' Quart. Jour. 

 Micro. So./ July, 1872, pp. 211 et seq. 



