WORMS. 



197 



IIG. 180. — Development of the vermiform embryo 

 of Dieyema; en, axial cell; g, germ cell: n, 

 nucleus. 



is a uterus ; for in it the germ cells, or ova, are lodged, and it is here that all the known 

 Stages of development are completed. The fully-formed embryos gscape from the 

 parent by pushing their way out between the cells of the epithelial envelope. 



The reproductive cycle of these worms has not yet been fully ascertained; but 

 some very interesting portions of it have been made known by the investigations of 



Van Beneden and Whitman. As Kolliker first 

 pointed out, the Dicyemids produce two very 

 distinct kinds of embryos, which he distin- 

 guished by the terms vermiform and infusori- 

 form. The vermiform embryo (Fig. 180, F) 

 develops directly into the parent form without 

 metamorphosis. The fate of the infusoriform 

 embryo (Fig. 181) still remains a puzzle ; but 

 it may be safely assumed that it either repre- 

 sents a male individual or a special form which serves to carry the species to new 

 hosts. In the latter case, as suggested by Balfour, it is not improbable that in the 

 course of a free existence, it may develop into a sexual form, the progeny of which 

 are destined to complete the cycle of development by 

 becoming again parasitic in the renal organ of a cepha- 

 lopod. 



Fig. 180, A to F, represents the more important 

 phases in the development of the vermiform embryo. 

 From the germ cell (A) arises, by division, a two-cell 

 stage (B), then a four-cell stage. One of the four cells 

 (en) now remains passive, while the other three go on 

 dividing (C, D), and arrange themselves so as eventually to completely inclose the 

 passive cell (E), which thus becomes the axial cell. The axial cell elongates as it 

 becomes enclosed, and from its two ends two cells {g) are split off at an early date, 

 which give rise by division to the germ cells. The embryo (F) attains nearly the 

 adult form, and is clothed with cilia before escaping from the parent. 



The infusoriform embryo, seen in Fig. 181, is somewhat pyrifoi-m, with the broader 

 end directed forward in swimming. It has a complicated structure, the significance 

 of which is entirely unknown. At the anterior end are seen two refractive bodies {r) 

 which lie above an organ called the urn {it). The urn consists of a wall (u), and a 

 lid (T), and contains four polynuclear cells (gr). The wall of the urn is hemispherical, 

 and composed of two halves. The lid is made up of four cells. 



The cells that form the posterior extremity of the embryo are ciliated. The germ 

 cells that give rise to the infusoriform embryos are larger and much less numerous 

 than those which develop into vermiform embryos. 



As the two kinds of embryos are never found simultaneously in the same parent 

 form, it has been supposed that there are two distinct adult forms, one of which pro- 

 duces exclusively vermiform embryos, the others exclusively infusoriform embryos. 

 The former have been called Nematogens, the latter Rhombogens. It has now been 

 ascertained that this distinction is not valid, the two forms being only two phases in 

 the same individual cycle of life. There appears to be two kinds of Dicyemids, how- 

 ever, one of which, so far as known, produce only vermiform embryos, the other pro- 

 duces first infusoriform embryos, and subsequently, after the escape of all these 

 embryos, vermiform embryos. 



Fig. 181. —Infusoriform embryo. 



