HOMOLOGIES. 139 



also drew a resemblance between the Nemertean reproductive organs and the " ovarian or female 

 series " in the leeches. So struck was this author by the differences between the Nemerfceans and 

 the Planarians (which he affirmed were only allied by the ciliated integument), that he proposed 

 to separate the former from the " true Turbellaria" under the name of the Cestoid Annelids. I 

 think, however, that we are scarcely warranted on structural grounds in making so radical a 

 change. 



Amongst recent writers, Dr. Cobbold, it appears to me on somewhat insufficient data, has 

 grouped the Turbellaria under the class Helmintha, which he conveniently widens to allow them, as 

 he thinks, to be near their allies the Trematoda. But it is to be observed that, while the Planarians 

 perhaps do approach the Trematoda, the Nemerteans diverge so much that the relationship is very 

 difficult to discover. The outline of the ovate and flattened Planarian somewhat resembles that 

 of the Distomes and their allies ; but there is nearly as much similitude between the former and 

 an Elgsia or Limaponiia, or again between a Sagitta and a Pish. The cutaneous texture of a 

 Trematode (for instance, Fasciola liepatica), according to Dr. Cobbold, is covered with minute 

 chitinous processes or spines, and is composed of an outer transparent epidermis, and an inner 

 fibrous cutis. In the Planarian, on the other hand, we have the ciliated epidermis and the 

 characteristic soft, cellular cutis, so conspicuous for its secretion and its tendency to diffiuence 

 under examination. " In the Fasciola the next layer is composed of numerous bands of 

 muscular fibres, in which four separate groups may be recognized more or less distinctly. They 

 have been described as so many layers, but they are not readily separated from one another." 

 Such is the description this author gives of the muscular system. In the Planarian the muscular 

 layers form distinct coats, which cannot be confounded, and moreover they seem to be formed 

 after a different type. I would, however, remark that in a transverse section of Ccmvpula oblonga, 

 Cobbold, a Distome from the bile-ducts of the Porpoise, there is below the chitinous cutis a 

 delicate layer of circular fibres — slightly indented by the bulbs of the chitinous spines, and having 

 a thin coat of longitudinal fibres underneath. Such therefore agrees with what Prof. Owen found 

 in Distoma clavatum. Dr. Cobbold also speaks of soft parenchymatous tissue filling up the 

 general cavity of the Fluke, and though not averse to such a disposition as a proof of further 

 divergence of type, yet in the Distome just mentioned (Campula oblong ci) transverse and 

 longitudinal sections show a complex arrangement of fibres and cells — only inferior to the more 

 differentiated muscular bands, fibrous tissue and cells, the presence of which in the Planarian is 

 so intimately connected with the physiology of the parts. In the case of the digestive system 

 there is apparently some analogy in form, since both Planarians and Plukes have branched, csecal, 

 alimentary organs, but then Vortex, and the whole of the Nemerteans to which the Planaria 

 Dendroccela are linked, deviate in a still greater degree from the parasites. The oral sucker 

 of the fluke has little homology with the protrusible proboscis of the Planarian, and still less 

 with the mouth of Vortex or the Nemertean. Moreover, the microscopic structure of the 

 digestive ramifications of the Planarian agrees very closely with the same organ in the 

 Nemertean, while it differs entirely from that of the fluke, with its " fibrous wall" and " columnar 

 cellules," or, as I should call them, papillae. Such differences probably depend much on the 

 divergent character of the food. Dr. Ehlers, in his arrangement of Worms, separates the 

 Nemertinea (Class V) from the Turbellaria, Ehrenberg, s. str. (Class IV), and interpolates the 

 round worms and Gephyrea between them and the Annelida. It is doubtful if the Gephyrea are 

 a higher type than the Nemertinea, and they certainly do not approach the true Annelida 



