I ANATOMY OF POLYCLADIDA 25 



faecal matter collects in the main-gut, and is discharged violently 

 by the pharynx into the water. In a few species, however, 

 the intestinal branches open to the exterior (Lang). Yungia 

 aurantiaca, a large and abundant Neapolitan form, possesses 

 such openings over the greater part of the dorsal surface ; 

 Cydoporus papillosus has marginal pores ; Oligocladus sanguino- 

 lentus apparently possesses an opening at the posterior end of 

 the main-gut ; and Thysanozoon brocchii frequently rends at this 

 point, in consequence of the accumulation of food. 



Respiration. The oxygen of the atmosphere dissolved in 

 the sea-water is, in default of a special circulatory fluid, brought 

 to the tissues of Polyclads in two ways. The ciliated epidermis 

 provides a constant change of the surrounding water, by which 

 the superficial organs may obtain their supply ; and the peri- 

 staltic movements of the digestive system, aided by the cilia of 

 the endoderm cells, ensure a rough circulation of the sea-water, 

 which enters along with the food, to the internal organs. The 

 papillae of Thysanozoon 'brocchii, containing outgrowths of the 

 intestinal branches, are possibly so much additional respiratory 

 surface, although still larger forms Bother Pseudoceridae) are 

 devoid of such outgrowths. 



Excretion. The excretory system of only one Polyclad 

 ( Thysanozoon brocchii) is accurately known. Lang, by compressing 

 light-coloured specimens, found the three parts of the system 

 known to occur in many Platyhelminthes : (1) the larger longi- 

 tudinal canals, and (2) the capillary vessels, which commence with 

 (3) the flame-cells in the parenchyma of the body. The mode of 

 distribution of these parts is not, however, ascertained. The canals 

 are delicate, sinuous, apparently intracellular tubes, coursing close 

 to the margin of the body and sending offsets which suspend the 

 canals to the dorsal surface, where possibly openings may occur. 

 In dilatations of these vessels bunches of cilia, and occasionally 

 flame-cells, are found. Usually, however, flame-cells occur at the 

 commencement or during the course of the capillaries, which are 

 straight, rarely branching, tubes of exceeding tenuity, and 

 appear (Lang) to be outgrowths of the flame-cells, just as the 

 duct is an outgrowth of a gland-cell. In fact there is little doubt 

 that the stellate flame-cells are modified parenchymatous gland- 

 cells, containing a lumen filled with a fluid into which a number 

 of cilia project and vibrate synchronously. The cells excrete 



