DEVELOPMENT OF POLYCLADIDA 



29 



incl their free-swimming young 



Acotylea indeed develop directly 



differ from Miiller's larva merely in the absence of the ciliated 



band and in the mode of swimming;. 



par 



FlG. 12. Section through 

 Miiller's larva of Thy- 

 sanozoon brocchii 

 (modified from Lang). 

 The right half is seen 

 from inside. x 150. 

 Semi-diagrammatic. 

 br, Brain ; dl, dorsal 

 ciliated lobe ; dr, saliv- 

 ary gland-cells of phar- 

 ynx ; e, eye ; ep, cili- 

 ated epidermis contain- 

 ing rhabdites ; mg, 

 stomach or main-gut ; 

 mg x , unpaired gut 

 branch over the brain ; 

 mo, "mouth ! ' of larva ; 

 n, n v section of nerves ; 

 oe, ectodermic pit 

 forming oesophagus of 

 larva ; par, paren- 

 chyma tilling the space 

 between the alimentary 

 tract and the body 

 wall ; ph, pharynx 



lying in the cavity of the peripharyngeal sheath, the nuclei of which are visible ; s 

 sl , sl-A, lateral ciliated lobes of the right side ; vl ventral ciliated lobe. 



Fig. 13. Diagrammatic transverse sections 

 of a larval Polyclad at different stages, 

 to illustrate the development of the 

 pharynx. (After Lang.) A, Larva of the 

 eighth day still within the shell. The 

 main-gut (m^) is still solid, the epidermis 

 is slightly invaginated, and a pair of mus- 

 cular mesodermic thickenings (ms) are 

 present. B, Young pelagic larva. The 

 epidermic invagination has deepened 

 and developed laterally. C, The lateral 

 pouches have formed the wall of the 

 peripharyngeal sheath, enclosing the 

 mesodermic, muscular, thickening or 

 pharyngeal fold {ph). (Compare Fig. 12. ) 

 Towards the end of larval life, when 

 the ciliated processes (si, Fig. 12) have 

 aborted, the stage D is reached. By 

 the opening outwards of the pharyngeal 

 sheath (ph.sh) the two apertures gm, or true mouth, and m, or external mouth, are 

 formed, which together correspond with the oesophageal opening of the younger larva. 

 (Compare the transverse section in Fig. 5.) 



Polyclads possess an undoubted mesoderm, which gives rise 

 to the muscles, the pharyngeal fold, and the parenchyma, The 

 ectoderm forms the epidermis, in the cells of which the rhab- 



