670 THE ANIMAL KINGDOM. 



A nephridial or excretory system is absent in Acoela. In other 

 Rhabdocoelida it may consist of a single median trunk opening posteriorly, 

 or of two lateral trunks which either unite and open posteriorly, or remain 

 independent, and then open ventrally or posteriorly, or, as is most usual, 

 into the pharyngeal cavity. It is rare for them to open near the centre of 

 the ventral surface, and to be connected anteriorly by a cross branch, from 

 which a canal passes backwards on each side parallel to the main trunk. 

 With these main trunks finer vessels are connected, which form a more or 

 less distinct network. Canalicules terminating in flame cells open into 

 this network, or into the blind terminations of its branches in Mesostoma 

 Ehrenbergi and a number of other Rhabdocoeles ( Vortex, Prostomum, 

 Monocoelis, &c.). Cilia may occur in the main or in the secondary canals. 

 In the Tricladida, Polycoelis nigra possesses both on the ventral and dor- 

 sal aspects a network of canals in which 'there is an undulating membrane. 

 Straight vessels lead to the surface of the body, and probably open upon 

 it ; and short canalicules ending in flame-cells are connected to the net- 

 work. In other cases there appears to be a pair of dorsal, or as in Gunda, 

 of dorsal and ventral, lateral canals, which anastomose anteriorly. These 

 canals open by more or fewer dorsal pores, which in Gunda are segmentally 

 arranged. There is a system of finer branching canals which end in flame- 

 cells. The excretory system of Polycladida is scarcely known. In Thy- 

 sanozoon, Lang detected large undulated canals which anastomose, and are 

 provided with stout lateral branches which approach the ectoderm, and 

 probably open externally. Fine branched canals are connected to the main 

 vessels, and end in flame-cells, and small cells with large cilia occur in the 

 course of the canals. It is certain that in some instances, and it is prob- 

 able that in all, the excretory canals are really intra-cellular 1 . 



All Turbellaria are hermaphrodite except the Rhabdocoele genera, 

 Microstoma and Stenostoma. But the male organs tend to ripen before the 

 female ( = successive hermaphroditism) in Acoela, in Polycladida, and 

 perhaps in others. The glands are paired with the exception of the ger- 

 marium in some Rhabdocoela. The testes in Rhabdocoela are two large 

 compact glands, in all other Turbellaria scattered and numerous globular 

 follicles. Specialised ducts are not developed in Acoela, Alloiocoela, and 

 some Rhabdocoela. The two vasa deferentia when present receive the 

 sperm from finer vessels, and either open together or separately into the 



1 The structures called by Geddes Pulsatella Convolutae, and found in the Acoelan Can-valuta 

 Schultzii, are perhaps not parasitic organisms, but indications of an excretory system. They occur in 

 very young specimens indeed. See Delage, A. Z. Expt. (2), iv. p. 150. According to Francotte, 

 the cross anastomosis of the lateral canals in front of the mouth has in Derostomum Benedeni an 

 external opening. As to Gunda, Lang states that the dorsal and lateral vessels are connected in the 

 septa of the body in which they are convoluted ; the efferent branches and pores are similarly 

 situated. He also observed in the same animal that some of the flame-cells were connected by basal 

 processes to the cells of the intestinal epithelium, and that others were actually situated among them. 



