200 PROCEEDINGS OF SOCIETIES. 



heads on two opposite surfaces in the cysts of the Quagga, 

 and Professor Huxley would like to have further confirmation 

 of his observations. 



In the Acanthocephali {Echinorhynchi) the head^ provided 

 with a spiny proboscis, is thrust through the wall of the 

 intestine of its host. There is no segmentation, and no ali- 

 mentary canal ; the genitalia are simple, and open in a large 

 posterior funnel. The integument exhibits an extraordinary 

 arrangement of reticulating canals, which arise very curiously. 

 It has lately been shown that in the ovary three shells or 

 coverings form around the ovum. The embryo, which is 

 directly developed, has four hooks, and is covered over with 

 spines. Those of fresh-water fish bore their way into the 

 legs of Gammari, and there lose their outer investments^ and 

 are left as mere sacs. A new blastema appears within and 

 develops into the chief organs of the worm, and touching 

 the wall of the sac at intervals, gives rise to the extraordinary 

 system of reticulate vessels. They are quite different, there- 

 fore, to the vessels of the water-vascular system. It is very 

 difficult to assign a distinct position to the Echinorhynchi. 



Lecture VIII. — The group Nematoidea was held to include 

 the Gordiacea, which in former lectures Professor Huxley has 

 kept as a distinct group. One of the most remarkable 

 features in Nematoides is the radial symmetry observable in 

 a cross section. It does not seem possible in them to distin- 

 guish dorsal and ventral surface, but there is a quadruple 

 arrangement round a centre, whilst the alimentary canal 

 presents in section the form of an equilateral triangle. In 

 this radial arrangement they approach the vermiform Echino- 

 derms. The cuticle is very thick and chitinous. Its lami- 

 nated layers, which cross and intercross, were till recently 

 mistaken for muscular layers. The integument is also very 

 largely perforated by pore canals. In these worms, too, 

 ecdysis is a constant phenomenon. Schneider, who has 

 recently written a great work on the group, states that twice 

 in the life of every nematod the skin is shed. Beneath 

 the thick cuticle is a cellular dermis, by which it is secreted. 

 This cellular dermis gives rise to four longitudinal ridges or 

 thickenings pi'ojecting inwards, causing those lateral lines 

 which have been so variously interpreted by different writers. 

 The two lateral thickenings a a are the most prominent, and 

 contain each a vessel of the water- vascular system. They 

 open by a pore placed near the oesophagus. Professor 

 Huxley, in an unknown species of nematod, observed that 

 the vessels were distinctly contractile, but no one has yet 

 confirmed this. However that may be, there are no cilia in 



