THE TREMATODA 55 



of segmentation in these is a blastosphere in which the outer layer of 

 cells becomes ciliated while the central mass becomes the " sporocyst." 

 He suggests that the first embryo of Gyrodactylus or daughter may be 

 compared with the former, the grand -daughter with the latter, as they are 

 both formed from a mass of embryonic cells which separate in the same 

 way as in Monostomum ; whilst v. Linstow (1892) has suggested that Gyro- 

 dactylus is a larval form capable of reproducing by an asexual method. 



Remarks upon the Order Heterocotylea. The general anatomy of 

 the group is sufficiently evident from the figures, and will be 

 treated, together with that of the Malacocotylea, later (p. 77). 



Reproduction. The life-history forms an important distinctive 

 character of the order. The only observed instance of copulation 

 is that of Polystomum (Zeller, 49), which is temporary, and the 

 permanent copulation of two Diporpae (Dujardin), which was shown 

 by v. Siebold to form that anomalous animal Diplozoon. 



From the fact that many of the Heterocotylea live isolated on 

 the gills of fishes, and that eggs are laid by them, it is probable 

 that self-fertilisation occurs ; this is borne out by observations on 

 Distomum, spp. by Looss (31), who finds spermatozoa within the 

 uterus before the external pore is formed. They could therefore 

 only have been derived from the ripe male organs ; and further, in 

 some species there is no penis, so that copulation could not have 

 taken place. If self-fertilisation may occur in Distomum, there is 

 every reason to expect its occurrence in the Heterocotylea. 



Each egg, when laid, consists of a single germ cell, derived from 

 the germarium, embedded amongst a considerable number of vitelline 

 cells (yolk cells) derived from the vitellarium, as in Triclads, and sur- 

 rounded by a shell, which is secreted by the walls of the "ootype." 

 The form of the egg, which is frequently of systematic value, 

 depends upon the shape of the " ootype." The shell is provided 

 with an anterior operculum. The shell substance is produced into 

 a filament at one or both ends ; that arising from the body of the 

 shell the " stalk " is used for attaching the shell to the host. 



Earely (Polystomum) the eggs are laid in the water. Practically 

 nothing is known of the segmentations and early stages in develop- 

 ment in the Order, though Zeller has described the course of events 

 in Polystomum. The egg cell becomes multinucleate before cell 

 division takes place ; ultimately a solid blastosphere is formed. 

 The yolk cells, meanwhile, become reduced in size ; and as 

 the growing embryo absorbs more and more of the yolk, the 

 yolk cells become broken down, and finally, when the embryo 

 acquires a mouth, the remains are swallowed. When hatched, 

 the larva swims freely in the water by means of -five incomplete 

 girdles of cilia, of which the three anterior are incomplete dorsally, 

 the two posterior incomplete ventrally (Fig. Y. 5, 6). Posteriorly 

 the body is produced into a caudal disc, armed with sixteen hook- 



