350 E. RAY LANKESTER ON STRUCTURE OF GREGARINiE, 



themonerian condition of Monocystis sipunculi, and as corre- 

 sponding to the " pseudo-filariuni" of Edouard van Beneden's 

 Gregarina gigantea. (See this Journal, present volume, 

 PI. XI, fig. 11.) I may mention here that the deceptive 

 resemblance borne by some young Gregarinae to small 

 nemaj;oid worms, and which suggested to Van Beneden the 

 very appropriate term ''pseudo-filarium " is not confined to 

 the monerian stage in all Gregarinae. Though it ajjpears that 

 in G. gigantea on the development of a nucleus the vermi- 

 cular movements cease, yet this is not the case in some other 

 Gregarinse. I have drawings of a monocystis from the 

 Annelid Leucodore ciliata which were of considerable larger 

 size than the " pseudo-filaria " of G. gigantea, and had a few 

 well- developed granules in the medullary substance. They 

 were exhibiting the most active, lashing movements, com- 

 parable to those of Nematoids, and fully explained the con- 

 fusion, by early observers, of Gregarinse with developmental 

 stages of those worms. 



The further development of these active Monerian forms 

 appears to be as represented in the figures 8 to 18. As a 

 matter of course one infers the existence of such a develop- 

 mental series from the observation of the association of the 

 supposed successive forms, not from direct observation of the 

 change of the earlier into the later phase. It is necessary to 

 state that the forms drawn were seen in sufficient number on 

 the same occasion, and on a sufficient number of occasions, to 

 warrant the conclusion that they really constitute a pro- 

 gressive developmental series. In addition to this the mode 

 of develoj^ment which they point to is quite in accordance 

 with the facts made out by Van Beneden with regard to his 

 G. gigantea. In fig. 8 we have the pseudo-filarium further 

 advanced with commencing nucleus in its anterior or broader 

 portion. In fig. 9 a constriction is seen pinching off the 

 actively vibratile "tail" from the broader " head" in Avhich 

 the nucleus is developing. In figs. 10, 11 the ''pseudo- 

 cercaria,^' as it may be termed, is represented. The posterior 

 or tail-piece retains its power of movement, whilst the head 

 has increased in bulk, and we thus have a minute organism 

 not unlike a tadpole or a Cercaria in form, urged along in 

 the field of the microscope by the lashing of its hinder piece 

 as they are by their tails. These pseudo-cercarise Avere found 

 in some numbers associated with small Gregarinse, such as 

 that seen in fig. 18. 



Bodies such as those draAvn in figs. 12 and 13 were also 

 observed and correspond Avith the separated "head" and 

 "tail" pieces of the pseudo-cercaria. It is obvious enough 



