268 ZOOLOGY SECT. 



which the " shell-glands " open. Actinodactyklla alone has a bursa 

 copulatrix (Fig. 205, b. c.). 



In the ordinary Cestodes each segment or proglottis contains a 

 set of reproductive organs similar to those of -a Trematode. There 

 may be a single genital aperture leading into a genital atrium, into 

 which both male and female ducts open ; or the male and female 

 apertures may be distinct. The testis is divided into numerous 

 minute lobes, from which proceed a number of fine canals joining 

 together to form the vas deferens, at the extremity of which is the 

 chitinous cirrus. There are two germaria, and either a single 

 vitelline gland, or two. The oviduct has its origin in a sort of 

 isthmus connecting the two germaria. It receives a narrow 

 fertilising duct from the receptaculum seminis, and then the 

 vitelline ducts, and becomes surrounded by a rounded mass of 

 accessory glands to form the ootype, which is not definitely enlarged. 

 Further forward it gives off the uterus. The latter is at first a 

 simple cylindrical outgrowth from the oviduct, but it usually 

 becomes large and may be extensively ramified. It has no external 

 opening in most instances, so that the eggs only escape from it 

 by the breaking down of the proglottis or by dehiscence. But 

 in some (e.g., Dibothriocephalus) it has an independent external 

 opening. The female aperture leads into a narrow canal the 

 vagina which ends in a receptaculum seminis from which the 

 narrow fertilising duct conveys the sperms to the oviduct. In a 

 small number of Cestodes the sexes are distinct. 



The development of some of the Platyhelminthes (Rhabdoccela, 

 Monogenetic Trematodes) is direct i.e., not complicated by the 

 occurrence of a metamorphosis ; in the Digenetic Trematodes, 

 the Cestodes, and some of the Planarians a metamorphosis 

 occurs. 



The eggs of the Polyclads, each of which consists merely of the 

 fertilised ovum (oosperm) usually enclosed in an egg-shell, are, 

 in most instances, laid in large numbers embedded in a plate or 

 capsule of slimy secretion. The ovum divides first into two parts, 

 then into fourl These are slightly unequal in size, one of them 

 being somewhat larger than the others, and by the position of this 

 and its relations to the other three the chief axes and planes of the 

 embryo are already determinable at this early stage. In the 

 notation adopted in following out the cell-lineage or order of 

 development of cell from cell in the embryo, this largest cell is 

 known as D : the other three are A, B, C the lettering following 

 the direction of movement of the hands of a clock when looked at 

 from above. As shown by subsequent changes, D is posterior, B 

 anterior, A and C are lateral. From these four cells (megameres) 

 are given off above in succession four sets or quartettes, each com- 

 posed of four cells. The members of the first quartette are desig- 

 nated la, 16, Ic, and Id, the small letter of each indicating derivation 



