ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 5()1 



phase, but by a. phase antecedent to the latter. In Dicyema trwicatum 

 and Microcyema vespa, found together in young cuttle-fishes, a larval 

 form gets into the urinary sac, fixes itself to the renal epithelium, and is 

 transformed into the " nematogene fondateur." This gives rise to the 

 primary Hematogenous forms by a process identical with that which 

 occurs in the multiplication of the vermiform phases. 



The larva has the general appearance of the ciliated embryo of the 

 nematogenous form of Dicyema, and the " nematogene fondateur " is like 

 the nematogenous form, but it has three axial cells instead of one, the 

 peripheral cells number twenty-eight, the polar cap cells eight, the para- 

 polar cells three, not two. Near the last in the larva are some cells with 

 corpuscles, probably of a nutritive nature, which disappear in the next 

 phase. 



The presence of three internal cells, disposed end to end, recalls a 

 female Orthonectid. In a still younger larva each internal cell is repre- 

 sented, as in the female Orthonectid, by a cell-germ which forms the 

 axial cell. So in the embryo of primary nematogens and rhombogens 

 a primitive cell-germ forms the axial cell. The axial cell is to be inter- 

 preted as a parthenogenetic ovum, the main mass of which is the primi- 

 tive cell-germ. The single polar body is reduced to a permanent nucleus, 

 which remains under the vitelline membrane to form, with the vacuolized 

 peripheral zone of the ovum, a follicle. The multiplication of the cell- 

 germs followed by their segmentation is a case of polyembryony. Thus 

 the peculiar phenomena exhibited by Dicyemids may be brought into 

 line, save as regards the persistence and secondary utilization of the 

 polar body. 



The larva described is the infecting agent ; it does not arise in the 

 Cephalopod, but its origin (from some other host) is unknown. The 

 infusoriform phase developed in the Cephalopod effects return to the 

 primary host. It seems probable that the Dicyemid type has been 

 evolved from the female Orthonectid, with the addition of a second 

 parasitic phase. 



New Species of Ptychodera.* — W. J. Dakin describes Ptychodera 

 pelsarti sp. n.. from the Abrolhos Islands, west coast of Australia. It is 

 the only species of the genus so far known from Australia. It is littoral, 

 and occurs in very shallow water. The colour is pale yellow, the body 

 is very translucent. The proboscis cavity shows the longitudinal 

 muscles gathered into distinct radially arranged bundles. The neck of 

 the proboscis contains a distinct well-developed but unlobulated 

 " racemose organ." The ventral part of the proboscis, the caecum, and 

 the " racemose organ " are compressed laterally. The cornua of the 

 nuchal skeleton are very long and reach the posterior half of the collar. 

 The body of the skeleton has a characteristic shape. There are two 

 proboscis pores. The stomochord has well-developed lateral pouches. 

 The oesophageal region of the pharynx predominates over the branchial. 

 The collar nerve-cord has a continuous lumen, and usually shows three 

 dorsal roots. 



* Journ. Linn. Soc. (Zool.) xxxiii. (1916) pp. 85-100 (2 pis.). 



