KOTICE OF THE GENUS TETRAPEDtA, ETC. 351 



that the anterior piece by further development of its nucleus 

 and general growth becomes a Gregarina, but the fate of the 

 mobile tail-piece seems to me more doubtful. 



Figs. 14 — 17 exhibit Gregarinee of the smallest size, in all 

 of which the process of longitudinal fission is going on. 

 This process does not appear to have been seen by Van 

 Beneden in his smallest G. gigantea, but occurs in all the 

 M. Sipunculi somewhat smaller than that drawn in fig. 18. 



Figs. 6 to 18 are drawn approximately to the same scale. 

 In fig. 14 the nucleus is doubled, probably the first step in 

 the process of longitudinal fission ; the other figures explain 

 themselves. 



The pseudo-cercaria, with its motile tail and motionless 

 head, is paralleled in Van Beneden's series by the motionless 

 cytod with its motile arm, though the condition of the parts 

 with respect to the development of a nucleus is not the same 

 in the two cases. We have them in the development of 

 Monocystis sipuncitli: — 



1. Pseudo-naviculae very similar to those of Monocystis 

 lumhrici. 



2. Monerian pseudo-filarian forms. 



3. Pseudo- cercarise with a motionless nucleated head and a 

 vibratile structureless tail. 



4. Monocystic Gregarina? multiplying by longitudinal 

 fission. 



5. Monocystic bean-shaped Gregarinse of -g-^th of an inch 

 in length. 



6. Free and encysted monocystic Gregarinse attaining a 

 diameter of ^th of an inch. 



Notice of the Genus Tetrapedia (Reinsch) and of Two 

 KINDRED New Forms. By William Archer. (With 

 Plate XXI.) 



Amongst " Unicellular" Algae (as well in the wider as in 

 the most strict sense), falling under the class Chlorophyl- 

 lacese) , forms with specially figured cells — that is, otherwise 

 than globular, ellipsoidal, or cylindrical, with more or less 

 abruptly or broadly rounded ends — are, as is well known, 

 numerous. One has only to call to mind, not to speak of 

 almost the entire family of Desmidiaceee, such forms as those 

 appertaining to the genera Polyedrium (Nag.), Scencdesmus 

 (Meyen), Pediastrum (Meyen), Ophiocytium (Nag.), Ccelas- 

 trum (Nag.), Characium (Al. Braun), Dimorphococcus (A.1. 



