VERMES. 581 



Gephyrean Thalassema Neptuni : haemerythrin replaces the haemoglobin 

 in some members of the Gephyrean family Sipunculidae. Colourless cor- 

 puscles of fixed outline occur in the vascular system of many Chaetopoda, 

 some Gephyreans, as well as in the coelome of some of the latter 

 (Priapulidae). 



No excretory or nephridial system has been discovered in Ckaeto- 

 gnatha and Acanthocephala, and, if present in Enteropneusta, it has a peculiar 

 form, unless the canals communicating with the coelomic cavities of the 

 collar and proboscis may be deemed to be metamorphosed nephridia. It 

 is peculiar also in Nemertea and Nematoda. But in other Vermes it falls 

 under one of two types, according as to whether it commences (i) by flame- 

 cells, or (2) by ciliated funnels. A flame-cell is a cell bearing a long 

 flattened but pointed cilium, which projects freely into a funnel-shaped 

 space with proper walls. The cell may or may not have basal processes 

 connecting it to the surrounding tissues. The funnel-shaped space is con- 

 tinuous at its apex with a tubular canalicule, by which it is connected to 

 the system of excretory vessels. It is said by Fraipont to have, in certain 

 instances at least, e.g. in Cestoda, a lateral opening into the coelomic spaces 

 of the mesoderm ; but the existence of this opening is denied by Pintner and 

 others. Such flame-cells are found in Trematoda, Cestoda, Turbellaria, and 

 Rotifera. The system of excretory vessels with which they are connected 

 varies in disposition to a certain extent. It may have, as in some Cestoda 

 and Ttirbellaria, numerous external apertures, which show a more or less 

 perfect segmental arrangement in certain of the latter (some Triclads) ; but 

 as a rule there are only one or two apertures variously situated and terminal 

 to the main canals. The vessels themselves seem to be intracellular, i.e. 

 to consist of a series of perforated cells, and cilia may occur in their course. 

 They may form a network, but in most cases there are two or more main 

 longitudinal canals, the terminations of which may be contractile. The 

 second type of excretory system occurs in Chaetopoda, Archi- Annelida, 

 Plirudinea, and in Gephyrea in a modified form. It begins with a series of 

 intercellular funnel-shaped apertures, composed each of a variable number 

 of cells bearing cilia. The funnels have primitively a segmentaL arrange- 

 ment, a pair to each somite of at least the middle region of the body, but 

 this arrangement may be lost or masked. They lead in Chaetopoda and 

 Archi-Annelida into tubes which may be in part intracellular, are some- 

 times convoluted, and have segmentally disposed external apertures. In 

 the Hirudinea the excretory tubes are also intracellular, but though their 

 external apertures are segmentally disposed, they form a network which is 

 either continuous or has become much restricted and discontinuous x . Two 

 sets of nephridia are distinguishable in Gephyrea except Priapulidae, where 



1 The presence of nephridial lobes in the medicinal Leech and its allies is' perhaps best explained 

 on the hypothesis that the lobes represent a restricted or disconnected network of vessels. 



