306 DR W. CARMICHAEL M'lNTOSH ON THE STRUCTURE OF THE 



stein,* in Germany; (Ersted,| in Denmark; Van Beneden, J in Belgium: 

 Claparede,§ in Switzerland ; and Delle Chiaje, |j in Italy. 



The confusion in regard to the structural characteristics of the order is well 

 illustrated in the descriptions given in the lately published " Catalogue of the 

 British Museum," and in the first chapter of Dr Cobbold's " Entozoa," where little 

 else than an array of doubts is produced as a solution of this question. In France, 

 again, the valuable Lectures of M. Milne Edwards,^[ for instance, are chiefly of 

 interest on the subject of the Nemerteans as stimulants for further investigation. 

 An examination of the discrepancies existing between the comparatively recent 

 and excellent researches of MM. de Quatrefages,** Max Schultze, Claparede, 

 Van Beneden, and Keferstein, demonstrate the same necessity for further 

 elucidation. MM. de Quatrefages, Van Beneden, and Keferstein have, per- 

 haps, gone more minutely than the others into the question, but all have 

 confounded the structure, or certain parts of the structure, of the Ommatopleans 

 with the Borlasians, whether one or both groups have been examined. M. de 

 Quatrefages investigated the Ommatoplean group more extensively than the 

 Borlasian ; while Prof. Keferstein paid more attention to the latter ; but he has 

 not entered so minutely into structural detail as the former, though his observa- 

 tions are, on the whole, more exact. Van Beneden likewise predicated of one 

 group what he had found in the other, and hence sometimes gave an erroneous 

 interpretation of the parts. While thus reviewing the labours of these distin- 

 guished naturalists, it must not be understood that I in the least degree under- 

 value their investigations ; but rather, that from a more continued series of obser- 

 vations,structures — about which they were in doubt — have been more clearly 

 determined, and many additional facts brought to light. Indeed, no one who is 

 acquainted with the patience and experience necessary for unravelling the anatomy 

 of these delicate creatures, will wonder at the occurrence of errors of omission or 

 commission, either in the labours of others or his own. Ever restless when alive, 

 prone to rapid dissolution when dead or too much pressed, and comparatively 

 few of the requisite transparency for examination, it is only by a happy com- 

 bination of circumstances that the structure of these animals can be successfully 

 demonstrated. 



One of the main objects of this paper is to show the essential differences 

 between the Ommatopleans and the Borlasians, from the skin even to the micro- 



* Zeitschrift fur wiss. Zool. xii. 1863. 

 | Entwurf einer Syst., &c, der Plattwiirnier, 1844. 

 J Memoires des Sc. des Acad. Roy. de Belgique, torn, xxxii. 1861. 

 § Recherches Anatomiques, &c, dans les Hebrides, 1861. 

 | Mem. sulle Storia, &c., vol. ii. Naples, 1825. 



•jf Legons sur la Physiol, et l'Anat. Comparee, torn. 5 me , pp. 460-65. 



** In his " General Outline of the Animal Kingdom," 3d edit. 1861, Professor Rymer Jones 

 strictly follows this author. 



