302 M. Krohn on the Anatomy and 



seen in one spot a spherical prominence, transparent as glass, and 

 projecting beyond the pigmentary envelope; it is perhaps the 

 cornea or the crystalline lens. At the circumference of the eye 

 a very great number of short fibrils are perceived ; in all proba- 

 bility these are fascicles of dehcate nervous fibres which spring 

 from the ganglion, and which seem to penetrate through the pig- 

 mentary envelope in the cavity of the eye. 



Conclusions. — After having passed in review the structure of 

 the Sagitta, we come at last to the question as to what place it 

 should occupy in the animal series. MM. Quoy and Gaimard, 

 who first noticed this animal, leave us in doubt on this point, and 

 they admit that they did not sufficiently examine its structure to 

 be able to pronounce an opinion. But even at present, when the 

 organization of the Sagitta is better known, it is difficult to arrange 

 it in a positive manner in any of the classes of our present 

 systems. It is certain that the Sagitta is no mollusk ; for al- 

 though its nervous system seems organized on the general plan 

 of these animals, most of the other parts of its organism and the 

 habits of the animal do not seem to justify this affinity. In 

 my opinion it can only be referred to the Annelides*. Here 

 again great difficulties present themselves ; for, not to mention 

 the absence of rings, and taking only a small number of the pe- 

 culiar characters of the Sagitta, where shall we find a genus of 

 Annelides provided with a hood and a similar cephalic armature, 

 fins, and so remarkable a disposition of the apparatus of genera- 

 tion ? Nevertheless it seems evident to me that the Sagitta can- 

 not enter into any other class than that of the Annelides, and 

 that we must consider it as an anomalous genus, until we shall 

 discover other animal forms which may connect it by gradual 

 transitions of organization with some known genus of Annelides, 

 or which shall completely separate it from this class of animals. 



EXPLANATION OF PLATE IV. B. 



In order not to encumber the figures with letters, we shall only indicate 

 a single organ or a single half of an organ, when there is a pair of them. 

 Figs. 3, 4, 5 and 6 represent the head magnified from ten to twelve times 



* Having had occasion to observe the Sagitta hipunctata during my last 

 voyage to Messina, I think I may say that I do not in any way share the 

 opinion of M. Krohn on the natural affinities of that animah I find nothirig 

 in its organization which can lead me to consider it as an Annelide, and I <'o 

 not doubt that it is a mollusk, having in certain respects a great analogy to 

 the Firolce. It seems to me that the part designated by the author under 

 the name o^ headh formed principally by the fleshy bulb of the mouth car- 

 rying the dental armature, and that it is the fold called hood in the prece- 

 di^ng memoir which represents the head. The ciuious arrangement of the 

 organs of generation noticed by M. Krohn constitutes the chief anomaly in 

 the structure of this animal. — Milne Edwards. 



