340 Dr. T. Williams on the Mechanism of Aquatic 



mentary organ in the Nemertinidae* is precisely conformable 

 to that of Tcsnia. In the former the whole organ has but one 

 external opening, which is situated, in form of mouth, at the an- 

 terior end of the body. In the latter each segment has its sepa- 

 rate opening (fig. 6, a) . In both the digestive diverticula are filled 

 with a corpusculated fluid. In both the spongy tissue, in the 

 midst of which the alimentary organ is lodged, is chambered into 

 capacious areolse in which the veal chylaqueous fluid is observed 

 iri'egularly to roll. In the Neraei'tinidse f, however, the entire 

 external cutaneous surface is ciliated. In the Cestoid worms no 

 vibratile cilia in any situation have been proved to exist. It is 

 a remarkable fact, that in all Entozoa the ciliaiy variety of epi- 

 thelium is entirely suppressed. The agency of these organules 

 is not essential therefore to the process of cutaneous respiration. 

 The Trematoid Entozoa are allied to the Cestoid by the most 

 intimate affinities of structure. In the former type as in the 

 latter, there exists but one orifice to the alimentary system. 

 The gastric diverticula, which in several species amount only to 

 two in number, in all Trematoid orders terminate csecally. These 

 caeca are filled internally by a corpusculated, and surrounded ex- 

 ternally by a non -corpusculated fluid. The oscillations of this 

 latter fluid have been observed by Nordmann, Duges, Rudolphi, 

 Blan chard, and others. It constitutes the true chylaqueous sy- 

 stem. Such a development of the blood-proper system in the 

 Trematoid Entozoa is opposed unqualifiedly by the analogy of 

 the whole Planarian family, after whose type the former are con- 

 structed. The parenchyma, which intervenes between the di- 

 gestive system and the external surface in these worms, is loose 

 and large-celled — readily traversed by fluid. This fluid reaches 

 the cutaneous surface ; it embraces and laves everywhere the 

 digestive caeca. The Trematoid Entozoa then, like the Cestoid, 

 respire cutaneously. Every spot of the surface is utilized. The 



* These Annelids are abundantly illustrated on our coasts by the genera 

 Borlasia, Polia, and Lineus. 



t The author is anxious here to bear witness to the excellence of the 

 memoir by M. Quatrefages on the Nemertine Annelids (Annales des Sci- 

 ences Naturelles, t. vi. 1846), with which he has only just become acquainted. 

 While his essay is acknowledged as a master-piece of minute descriptive 

 anatomy, the author is constrained to ditfer toto ccelo from M. Quatrefages 

 in the physiological interpretation of parts. In these worms the French 

 naturalist indicates correctly the mechanism of the respiratory function : — 

 " Si c'est la surface entiere du corps qui joue le role d'organe respira- 

 toire, il s'ensuit que la respiration doit s'exercer principalement et plus im- 

 mediatement sur le liquide qui remplit les grandes cavites que sur le sang 

 lui-meme, puisque celui-ci est renferme dans des vaisseaux qui sont en- 

 toures par le liquide dont nous parlons." — Op. cit. p. 269. 



