ANNULOSA. 439 



metamorphosis. The embryo has at first a ciliated non- 

 contractile, oval body, exhibiting no structure but a semi- 

 lunar superficial cleft, provided with raised edges. After 

 a time, a small actively-contractile, vermiform creature, 

 resembling the parent, escapes from the interior of the 

 larval form, which it leaves behind like a cast skin. The 

 semilunar cleft becomes the mouth of the imago, and is 

 only part of the larva carried away. The Gordiacel l best 

 enable us to connect the Turbellaria with the very puzzling 

 group Nematoidea, and the structure of several species 

 belonging to the two genera which compose the group 

 Mermis and Gordius, have recently been made the subject 

 of two elaborate monographs by Meissner. 2 



The Gordiacel are excessively elongated, thread-like 

 worms, plentiful enough in Thames mud, and, as Yon 

 Siebold discovered, partake both of the free habit of the 

 Nemertidce, and of the parasitic nature of the Nematoidea. 

 The young Mermis, for instance, is parasitic upon insects, 

 inhabiting the perivisceral cavity of the larva or of the 

 imago. 



The Nemertidw seem to differ from their close allies the 

 Turbellaria, in possessing a vascular system distinct from 

 and added to the water- vessels. In the Hirudinidce, Leeches 

 and Earthworms, a system of vessels homologonous with 

 the pseud-haemal system exists, and, in addition a series 

 of more or less coiled tubules lie in the perivisceral cavity, 

 and open, by pores, on the ventral surface of the body. 

 These organs have been regarded sometimes as secretory, 

 sometimes as respiratory apparatus; but all that we 

 know about them in reality is that they are tubular, and 

 are more or less richly ciliated within, and that, in some 

 cases (Nais, Lumbricus), they present at their internal ex- 

 tremities a ciliated aperture, whereby they freely com- 

 municate with the perivisceral cavity. 



There remains, however, yet another system of vessels 

 in the Annuloida the ambulacral vessels of the Echino- 

 dermata. These are frequently termed " water- vessels," 

 and, indeed, if we regard the structure with reference to 



(1) Huxley, General Natural History. 



(2) Beit-rage zur Anatomic und Physiologic von Mermis Albicaus, v. Zeitschrift 

 fur Wiss, Zoologie, Bd. v 1854 ; and Beitrage zurAnat. & Physiol. des Gordiacen. 

 Ibid. Bd. vii. 1855. 



