THE TREMATODA 



veneris, Vogt ; and others, from body cavity of Sagitta, Salpa, 

 F"elella, etc. 



In addition to a number of anatomical peculiarities in the 



FIG. XV. Development of Embryo of Distomum tereticolle. (From Schauinsland.) 



1. The living egg with its gelatinous envelope (a) ; b, egg shell deposited in ootype ; c r 

 germ cell with large nucleus ; d, vitelline cells ; outlines not distinguishable in living egg, but 

 the seven nuclei indicate the number of cells. 



2. Segmentation of the germ cell has resulted in a solid blastosphere ; the yolk is greatly 

 reduced. One of the blastomeres (/) at the upper pore is larger than the rest. 



3. This cap cell has divided and given rise to three cells, two at one pole, one at the 

 other. The yolk is nearly absorbed and is not represented in this figure. 



4. The cap cells have flattened out and form a "yolk envelope," investing the embryo and 

 the remains of the yolk ; two nuclei are seen above, two below, and one on the left side. 



5. The embryo, after separation of the yolk envelope, has become differentiated into 

 an outer layer of flat cells or ectoderm (<?) and a central mass (e) ; four nuclei of the shell 

 membrane are seen ; ft', operculum. 



6. Longitudinal section of the larva or miracidium of Distomum hepaticiim. (From Coe in 

 Zool. Jahrb. ix. 189(3 (Anat.), p. 561.) g, flat epidermal ciliated cells, derived from g in Fig. 5 ; h, 

 the underlying cellular cutis ; i, head glands (Coe), one on each side of the body, each with a 

 narrow duct opening on the tip of the head papilla, which is represented partially retracted ; the 

 secretion is indicated by a row of dots ; j, the vestigial enteron ; fc, eyes, resting upon the 

 brain (t) ; in, flame cell, on each side, whence a fine duct (n) passes "backwards to open to the 

 exterior at (o) excretory pore ; p, general parenchyma ; q, a germ ball ; r, germ cells, posteriorly, 

 lying in a cavity which appears to represent the coelom. 



7. Transverse section of the embryo at the level of the eyes, g, ectoderm cell, with its 

 peculiar elongated filamentous nucleus cut longitudinally. In Fig. 6 the nuclei are cut trans- 

 versely, and appear in the hind end of each cell. Other letters as before. The nuclei outside 

 the eye are possibly part of a sensory apparatus. Coe was unable to see the retractor muscles 

 of Leuckart. 



nervous and reproductive system, to which reference is made 

 below, the most striking and interesting differences between the 

 Heterocotylea and the Malacocotylea lie in the developmental 

 history, to illustrate which a concrete example may be described. 



