228 REPORT— 1851. 



certainty that they do not enact any part concerned in the reproductive func- 

 tions. With the organs devoted to the hitter uses they exhibit no sort of 

 communication. The structure of the cesopliageal parietes are coloured by 

 no pigment. The intestine proper, however, coincides to the Annelidan law, 

 and displays the brightest yellow colour, streaked in every direction by the 

 plexiform blood-vessels, in the meshes of which the biliary gland-structure is 

 lodged. A correct conception of the ultimate structure of intestinal parietes 

 may be derived from a close examination of the figure of the circulation of 

 Arenicola, formerly given (Plate III. fig. 7,/). At the commencement of the 

 posterior third of the body the digestive canal of this worm loses its enteric 

 character. The liver-enveloped tube degenerates into a smooth-walled, 

 straightened canal, stretching under this form from this point to the tail ; 

 the traces of segmentation so general in the digestive system of the Annelida 

 are here scarcely perceptible. Although destitute of biliary glandular 

 tissue in its coats, the posterior straight segment of the canal is surrounded by 

 a curious loose flocculent tissue, which both Lamarck and Cuvier have mis- 

 taken for a part of the reproductive system. Each elementary process of this 

 tissue consists ofashigle vessel, terminating in a culde-sac, and enveloped in a 

 layer of nucleated cells ; they project to the distance of about the eighth of 

 an inch from the exterior surface of the intestine ; they are also attached to 

 the interior surface of the integumentary cylinder, a few extending from the 

 one to the other. Milne-Edwards defines them as "appendices secreteurs 

 de la matiere faune, excretee par la peau " (vaisseaux biliares ?). Lamarck 

 has described them as ovaria. As already stated, they consist of a single 

 vessel (not a looped vessel), around which a layer of nucleated cells clusters. 

 If these cells produce anything, it must enter the blood-channel, which forms 

 the axis of the process, and thence mingle with the blood. If they do 7iot, 

 the blood-vessel must be regarded merely as a collateral receptacle into 

 which the blood may rush during the contortions of the animal, and as bear-' 

 ing some analogy to that system of vessels which surrounds the lungs of the 

 cetacean mammalia. If they are glands, they are most certainly destitute of 

 excretory ducts. If they are designed to supply a fluid tributive to diges- 

 tion, it is anomalous that they lie disposed over the hindmost segment of the 

 digestive canal. There is ho character detectible in the structure of the cells 

 suggestive of their true function. Neither these appendages, nor the pyri- 

 form diverticula attached to the oesophagus (Plate XI. fig. 52c), can in the 

 present state of knowledge be physiologically defined. It is not easy to ex- 

 plain, within the limits of the same class of animals, why the same secretion 

 should proceed in different species from organs so remarkably dissimilar in 

 structure. Between these pouches in Arenicola, for example, and the salivary 

 glandules which beset the proboscis and oesophagus of nearly all other 

 Annelids, there exists no homological affinity. Analogy therefore affords no 

 ground whereon to rest the supposition that their secretion consists of saliva. 

 Neither can it consist of bile, for already an extensively diffused gland is 

 furnished for the production of this fluid. It admits of no denial that special 

 necessities arise in some species of animals, from the nature of the food,. 

 obliging the provision of some special organ to supply peculiar wants. The 

 physiological signification of such organs cannot be explained on " general 

 principles." The peculiar want must first be understood, the peculiar cor- 

 relate of that want may then be defined. 



In Arenicola the cutaneous system is very profusely supplied with mucus- 

 producing follicles. The slippery secretion by which the animal is externally 

 covered, must very materially facilitate its progress through the sandy soil 

 of the shore, by diminishing the friction between its sides and those of the 



