Prof. P. J. Van Beneden on the Intestinal Worms. 349 



announce that the hooks of the embryo are still to be found in 

 the sac of these cystic worms. All Cestoids have such a vesicle 

 at first ; but in some species it continues very small, whilst in 

 others it is more and more developed, and attains a remarkable 

 size. In the Tcenice it falls off when the tape-worm has found 

 its final resting-place ; but in Tetrarhynchus it persists. The 

 vesicular or cystic worms ai'e by no means morbid and accidental 

 modifications of tape-worms ; the tape-worm which is developed 

 in a carnivorous animal must first have lived as a vesicular worm 

 in another herbivorous animal. The vesicular worm is the pro- 

 scolex, the head of the tape-worm the scolex; and by genital 

 propagation and growth the so-named joints are formed, which 

 represent the highest and perfect form of the species. 



The third part of the work contains investigations respecting 

 certain Nematoids, as Mermis nigrescens and Echinorhynchus 

 acus. With the exception of Filaria Mustelarum, Rud., from the 

 lungs of the pole-cat (pi. 23), and Prosthecosacter inflexus, 

 Diesing, from the wind-pipe of the narwhal, all the Nematoids 

 treated of by V. Beneden are from the class of fishes {Proleptus 

 gordioides, V. Ben., Spiropterina coronata, V. Ben., three species 

 of Dacnites, and CucuUanus elegans). 



Mermis nigrescens, a thread-worm, which once, in the begin- 

 ning of summer, after a stormy night, appeared suddenly in 

 astonishing quantity in the gardens of Louvain, lives as a para- 

 site in the cockchafer. Probablj', in consequence of the rain, 

 the worms were expelled from these insects. All the individuals 

 were female. The black colour of these thread-worms is to be 

 ascribed to the eggs. In the eggs the embryo may be already 

 distinguished ; and by cautious pressure the shell was success- 

 fully burst, and the embryo set at liberty to move freely. Mer- 

 mis is thus viviparous ; the embryo has already the form of the 

 perfect animal. There is no metamorphosis, in the ordinary 

 sense, in the Gordiacea or the thread-worms, any more than 

 digenese. Neither does digenese appear to occur in the thorn- 

 headed worms. The observations, however, are hitherto far 

 from numerous, and even not quite complete. 



The fourth part of the work is dedicated to the theory of pro- 

 pagation by means of alternating stages of generation. Within 

 the last few years, few terms have been more frequently repeated 

 in physiology and embryology than that of "^ GenerationswechselP 

 Since 1842, when the Danish naturalist Steenstrup, then a 

 young man, and unknown beyond his own country, published 

 his Academical Essay on that subject, a multitude of works, of 

 greater or less extent, on the same subject have seen the light. 



