xii PREFACE. 



something of arbitrary selection with respect to the position of 

 osculant groups in a systematic arrangement. 



To the citations on Bryozoa add ALLMAN, Monograph on the Fresh-water 

 Polyzoa, Ray Soc. 1856. 



In the class of Entozoa the most important discovery of modern 

 times consists in the demonstration that the Cystica are only imper- 

 fect conditions of Cesto'idea, of Tcenia. It is hoped that the English 

 reader will be satisfied with the important additions at pp. 173 

 176, and p. 181. It must not be omitted however that the state- 

 ment of V. SIEBOLD, p. 175, that the different forms of Coenurus and 

 Cysticercus depend upon the locality assumed by the embryo and 

 not upon original difference of the Tcenice of which they are the 

 the progeny, is not tenable. For LEUCKART has found that though 

 Cysticercus pisiformis, C. tenuicollis and Ccenurus cerebralis are all 

 developed into Tcemce, called collectively Tcenia serrata, when 

 swallowed by a dog, yet that the first and third at least of these 

 species certainly differ in the number and form of the booklets and 

 in other respects ; and also that a sheep is not rendered vertiginous 

 by swallowing the eggs of Tcenia solium, or of T. pisiformis. 

 Hence Tcenia solium and T. serrata are different species. 



KOELLIKEK in his Zeitschr. f. Wissensch. Zool. IX. 1857, p. 139, confirms, 

 in the scolex of a tape- worm from Murcenophis saga, the existence of many 

 free openings externally of the vascular system already observed by 

 WAGENER (Nov. Act. Nat. Cur. xxiv. Suppl. pp. 16 and 33), as stated by 

 KOELLIKER. 



It has been already related, p. 188, that Pentastoma ought to be 

 removed from the Entozoa and placed amongst the parasitic Crus- 

 tacea Lernceacea. See chiefly the memoir of the first discoverer of 

 the young form, Prof. VAN BENEDEN, cited p. 625. 



It appears that Prof. VAN DER HOEVEN is of opinion that the 

 Acanthocephala, p. 184, would stand more properly after the Trema- 

 toda, p. 188, intermediate between these and the Nematoidea. 



With respect to the place of the Rotatoria in the systematic 

 arrangement, many memoirs have been recently published in 

 V. SIEBOLD u. KOELLIKER'S Zeitschr. f. wissensch. Zool. by LEYDIG 

 (vi. 1854), C. VOGT (vn. 1855), and H. BURMEISTER (vin. 1856). 

 But the result of all these discussions seems to lead to no other con- 

 clusion than that they form a class of low-developed, articulate ani- 

 mals, allied to the ringed worms and the inferior forms of Crustacea, 



