SYSTEMATIC POSITION OP A GIGANTIC EARTHWOEM. 71 



is single, and gives off on eitlier side a pair of large " hearts," uniting it with the 

 ventral vessel. 



The existence of a double dorsal vessel appears to be an embryonic character, inas- 

 much as the single dorsal vessel of Lumbrkus and of Criodrilus is formed by the coales- 

 cence of two vessels, at first distinct \ 



Alimentary System. 

 The alimentary canal commences by a large pharynx occupying the anterior three or 

 four segments of the body, which is firmly attached to the body-wall by innumerable 

 muscular bands ; the oesophagus is long and narrow, and, as in most other intra- and 

 postclitellian Earthworms, is continued beyond the gizzard; m Lumbriciis the gizzard 

 marks the posterior boundary of the oesophagus. Two or three segments behind the 

 gizzard is a peculiar dilatation of the oesophagus (see fig. 4/, PI. XV.), conspicuous by 

 its reddish-purple colour, which is caused by the presence of a rich, network of blood- 

 vessels almost entirely covering its surface ; on slitting open this section of the oeso- 

 phagus it was found to have the appearance represented in fig. 5 of PI. XV. ; the 

 epithelium is extremely thin and tifinsparent, and covers an extraordinarily developed 

 plexus of blood-vessels, composed of larger trunks running from end to end, and con- 

 nected here and there by small transverse branches ; no trace of the vascular network 

 could be detected in the rest of the oesophagus — the network disappeared entirely at the 

 junction of this dilatation with both the preceding and succeeding sections of the 

 oesophagus. The existence of a vascular plexus, or rather sinus, within the coats of the 

 alimentary canal has been recorded by Vejdovsky in the Limicolous family Enchy- 

 trseidse; in these worms the dorsal vessel, instead of running continuously from one end 

 of the body to the other, ceases to be visible at the commencement of the intestines, 

 and in fact enters the tissues of the organ at this point and forms a continuous sinus 

 entirely surrounding the gut, but lying between its two muscular coats. In the same 

 memoir Dr. Vejdovsky describes two csecal diverticula from the commencement of the 

 mid gut, exactly at its junction with the oesophagus, which are similarly furnished with 

 a network of vessels within their substance ; these are compared to a dilatation occurring 

 at the hinder end of the oesophagus in Enchytrwus veiitriculosus ^, which appears to be 

 very closely similar to the structure I have just described in Microchceta ; the walls 

 are specially thickened and contain an immense number of clear vessels, which 

 Dr. Vejdovsky presumes, from analogy, to be blood-vessels; he does not particularly 

 describe the minute structure of this vascular dilatation, merely remarking its probable 

 homology with the paired diverticula of other species of Enchytrceus; Dr. Vejdovsky 

 furthermore suggests that this organ functions as a liver, and corresponds morpho- 

 logically with the paired cseca so commonly found in the genus Ferichceta. 



' Balfour, ' Comparative Embryology,' vol. i. p. 282. 



' ' Monographie der Enchytraeiden,' Prag, 1879, loo. cit. pi. vi. fig. 4, Idi; pp. 33-35. 



VOL. XII. — PABT III. No. 2, — August, 1886. jj 



