204 DESCRIPTIONS OF PREPARATIONS. 



these points. In Urochaeta and Criodrilus the dorsal and supra-neural vessels 

 give origin to the integumentary capillaries. Dilatations occur in the nephridial 

 capillaries, especially in the middle and posterior regions of the body. Similar dila- 

 tations occur elsewhere. 



The most remarkable variations in the circulatory system of Oligochaeta 

 are the following. The dorsal vessel, in two sp. of Acanthodrilus from New 

 Zealand is double from the pharynx backwards, an embryonic feature seen also 

 in some Polychaeta : in Megascolex and Microchaeta the sections of the dorsal 

 vessel in the anterior somites are double between the septa, single where they 

 perforate them (Beddard, Proc. Roy. Phys. Soc. Edinburgh, 1885). In some 

 Earthworms, e. g. certain sp. of Acanthodrilus, Urochaeta, the posterior hearts 

 connect a supra-intestinal vessel to the supra-neural or ventral vessel ; and in some 

 instances, e. g. Urochaeta, Enchytraeidae, &c., the dorsal vessel has moniliform 

 pulsatile dilatations, and in the Lumbriculidae blind lateral and sometimes branched 

 processes. Integumental capillaries are wanting in aquatic Oligochaeta, except at the 

 posterior extremity of the body in some Naidomorpha, the Tubificidae, and Criodrilus. 



The blood-vessels of Lumbricus are lined by an endothelium, the cells of which 

 vary in character in different regions, suggesting a distinction into arteries and 

 veins. The contained liquid is coloured with haemoglobin, as in most Oligochaeta, 

 except Aeolosoma, Chaetogastridae, and most Enchytraeidae. It contains floating 

 corpuscles, flattened, fusiform, sometimes nearly circular, which, according to Ray 

 Lankester, are ' the nuclei of the endothelial cells set free from the walls of the 

 vessels.' Vejdovsky, however, points out that the Lumbricidae, like the Leeches, 

 have minute valve-like or irregular masses of cells connected to the walls of the 

 vessels by fine processes. They are also present but confined to the dorsal vessel 

 in aquatic Oligochaeta. Some of these cells are set free, according to him, and form 

 the corpuscles which are absent only in Enchytraeidae and Naidomorpha. 



The coelomic fluid contains amoeboid cells, ' large colourless vacuolated cor- 

 puscles, with a ragged outline often produced into filaments and provided with a 

 large nucleus.' 



The excretory nephridia, or segmental organs as they used to be called, are 

 found one pair to each somite throughout the whole extent of the body from the 

 fourth somite onwards. Each organ consists of a ciliated funnel or nephrostome, 

 a convoluted tube, and a terminal muscular duct. The funnel is bilobed, but 

 one of its lips is very small : its free margin is formed by a single row of ciliated 

 cells. It opens into the cavity of the somite anterior to that in which the duct 

 opens externally \ an arrangement common to all Oligochaeta, but by no means so 

 in Polychaeta. The tube, therefore, to which it is connected passes backwards 

 through a septum. The convoluted tube forms typically three loops. The first 

 portion of it is clear-walled and ciliated; the second rather wider, its cells 

 granular, and their cilia peculiarly long ; the third part is wider still, its cells 

 coarsely granular and not ciliated. The tube appears to perforate the cells, i. e. is 

 intra-cellular as in the corresponding region of the Leech's nephridium. The duct 

 is long, with muscular walls, and is dilated. It corresponds to the more or less 

 pyriform vesicle into which the glandular part of the nephridium opens in some 



1 The Oligochaete Plutellus is an exception to this rule. Its nephridia (and oviducts) open 

 both externally and internally in the same somite, as in many Polychaeta. 



