140 HOMOLOGIES. 



more closely. Prof. Huxley, again, groups the Nemerteans amongst the Scolecida, charac- 

 terising the " water- vascular system" of this heterogeneous class as having ciliated tubes 

 throughout. This, of course, cannot apply to the Nemerteans, and not even to the Planarians. 



The branched water-vascular system (which Prof. Owen regards as excretory) of the Pluke 

 has no counterpart in the Planarian, and cannot be supposed to be closely allied to the vessels of 

 the Nemerteans. 



A decided difference is apparent in regard to the nervous system, which is much more 

 conspicuous in the Planarian than the Pluke ; indeed, observers who are familiar enough with 

 other parts of the structure of some species of the latter have not seen such at all. It is 

 described by Prof. Owen in Distoma clavatum as in the form of a pair of cephalic ganglia 

 connected together by a thin commissural filament above the pharynx, and giving off two 

 main lateral nerves. Two much larger ganglia occur in the Planarians, connected by a broad 

 commissure, and the branches to the surrounding parts are more distinctly arranged. Prof. 

 Owen states that pigment-specks, called "eye-specks," are present in the Polystoma of the 

 urinary bladder of the toad and frog, as in the locomotive ciliated larvae of most Trematoda ; but 

 as a whole the special organs of sense are much more highly developed in the Planarian. 



In regard to reproduction there is some resemblance between the groups, both Flukes and 

 Planarians having male and female organs developed in the same animal. Both are oviparous, 

 and the ova produce ciliated embryos; but the young of the fluke soon lose the cilia, and 

 represent only the first stage of a series of metamorphoses which occur before reaching maturity. 

 The embryo of the Planarian, while, perhaps, undergoing metamorphoses in some cases, comes 

 out of the egg in others nearly in the same form as the adult, and never loses its cilia at any 

 period. Besides, too much reliance cannot be placed on this common metamorphosis, for we may 

 as readily arrange the Echinoderms with the Nemerteans on account of the Pilidium- development, 

 as class the Planarians with the Trematoda on this account. 



The habits and motions of the two groups, it is well known, are widely different. 



Having thus indicated some of the chief points of divergence and affinity between the 

 Planarians and the Trematoda, we may now examine the relationship between the former and the 

 animals with which we have more particularly to deal, viz. the Nemerteans. 



In regard to the general structure of the cutaneous textures there is much resemblance. Both 

 have a ciliated cuticle, a soft, easily disintegrated cutis, chiefly composed of cells and areolae, and 

 capable of secreting abundant mucus. In the skin of no Nemertean, however, have I seen any 

 urticating or " stabformigen" bodies. 



The muscular coverings are similarly grouped into definite layers of longitudinal and circular 

 fibres. On the ventral surface of the Planarian, however, we sometimes have an inner layer not 

 represented on the dorsum, a fact that has been overlooked by Professor Keferstein in his recent 

 valuable remarks on the Planarians. 1 



The digestive systems, though apparently divergent, are really allied in an intimate manner. 

 The mouth in the Planarians follows the habit of the organ in the Anopla in opening behind the 

 ganglia, but more posterior in position. The large proboscis in the Planarians is probably homo- 

 logous with the oesophageal division of the digestive tract in the Nemerteans ; and in the typical 

 Idneidce amongst the latter the oesophageal region is frequently everted during feeding in the form 



1 f Beitriige zur Anat. u. Entwicklung. einiger Seeplanarien/ &c, 1868. 



